Smash 4 doesn't "need" to be modded, because it isn't about should/shouldn't, or need/needn't, but of can/can't.
I ended up writing a thesis on the subject (because I have zero discipline as a writer), and as a result of its length, I'll collapse it via spoiler tags; the tl;dr version will be found below.
[collapse=On Modding, Content, and the Relationship between Creator and Consumer]
In my view, game modding seems categorically similar to fan fiction, despite the disparity of the media involved. You take a fictional universe or narrative that you're passionate about, then personalize the experience further via your own writing (or fan comics, or fan art, or whatever). Likewise, you take a game that you're passionate about, then personalize the experience further via modding and/or hacks. And for both cases, you share your output with fellow passionate fans of the source material.
There are no legal or financial downsides for the creator of the source material, because fan fiction and mods alike aren't sold for profit (unless they are, in which case you have a violation of copyright laws). The creator isn't making money, necessarily, but they aren't losing any either -- unless you consider not making money to be equivalent to losing revenue potential. But only a minority of the consumer base would engage in fan fiction/modding/other forms of fanmade products to begin with (unless this minority becomes so substantive that it noticeably hurts revenue, in which case things become problematic (though that's another discussion)).
Evidently, the ideal is to support the creator with your fistfuls of cash, but a minority do circumvent this for X reason(s), which can't be helped. This doesn't make modding impermissible; it only makes you somewhat inconsiderate for not monetarily supporting the things you like and those who make them. But that's also another topic of discussion.
The point of this tangent is to whittle down the reasons why modding shouldn't be pursued, so far as Smash 4 is concerned. If legal and/or financial concerns aren't relevant to the discussion, then what other dissuasive factors remain?
Barring those concerns, we have to turn to art itself, and the creator-product-audience relationship trifecta. Is modding a game (X) an act of disrespect toward the creator (Y) -- they who have invested much time and effort in the creation of the game? Is modding Smash 4 an act of disrespect toward Sakurai, Sora, and Bamco -- they who have invested two or so years of development across two systems, with Sakurai in particular known to be singularly dedicated to his work, even in spite of health obstacles?
Is writing fan fiction about a book or a movie or a television show an act of disrespect toward the creator -- they who have invested much time and effort in the construction of their narrative (and the execution of this narrative within their respective media)?
Why would it be? In what way does Y follow from X?
If a product shouldn't be modified further by the audience, then where does this "shouldn't" come from? Is it because the audience would not be showing gratitude for the time and effort invested in the creation and dissemination of the end product? But for what more gratitude could a content creator wish than for their fans to be so passionate about their product(s) that they would explore the creator's original ideas in new ways and forms, as an extension, if not a continuation of the creative process?
Why would they be resentful of such a percentile of their audience? Because of loss of potential revenue? Those who truly do appreciate the creator's output will surely and certainly support them, monetarily or otherwise. Because they consider their work their "baby", and a violation of this sanctified artistic vision is a proverbial slap in the face? Your artistic vision is lost the moment you set your baby free into the world, where they grow and flourish in the imaginations of the audience.
As one who writes, I'd be elated to know that some of my (hypothetical) readers engaged in fan fiction based on my (hopefully eventual) works. Such activity has zero negative legal or financial repercussions for my person, and it can't shred my artistic ego to tatters, because the reason I write is to share ideas with others, to inspire them and take their imaginations to places never before seen -- not for the readers to glorify me as a writer, or to praise me for my efforts (though that would be a nice bonus, of course).
If Sakurai were to feel slighted in the light of Smash 4 modding, then that would be on him. The only thing the creator owes to their audience is the product, no more and no less; and the audience doesn't owe anything at all to the creator, since they have zero obligation to purchase or support the product. They can purchase or support the product (and thereby, the creator), but they don't need to.
Which brings us back full circle to my original point, and thereby, the conclusion of this exhausting and circuitous arrangement of ideas.
[/collapse]
At this juncture, it would seem that there are no reasons why we shouldn't mod Smash 4. There are also no reasons why we should.
Those that want to will, and those that don't will not. And this, for whatever reason(s).
Modding Smash 4 is by no means an indication that the modder thinks Smash 4 is broken, or needs to be fixed, or isn't a complete game, or so forth. Smash 4 will be a complete game with or without modding. Unless, of course, the modder does think Smash 4 needs to be fixed, and mods for that express purpose. But what does anyone else care if that's the case? If you think Smash 4 is good or perfect as it is, you wouldn't engage in the world of modding in the first place.
If there are those that want to mod Smash 4 to increase competitive and/or technical depth even further, in what would be a spiritual successor to P:M, then by all means. Those who have no interest in playing such a mod will stick to regular Smash 4, or try other mods (if they are made), or make their own.
Allow me to close this by quoting a sage Jedi master:
"Can or cannot. There is no should or need."
-Yoda
EDIT: At no point in this discourse I have made any notes on the practicality of modding, especially with the Wii U -- and this, because I'm not knowledgeable enough on the subject to offer any insights. Even so, my point would still stand; the fact that there are risks or obstacles in pursuing modding, doesn't make the practice of modding impermissible or unfavourable.