For a valuable secondary, it is really nice to have a few skewed MUs, since usually you are pulling them out against a dedicated main so you need some sort of advantage.
However, ease of play is also important. As is consistent gameplan. Mario makes a solid secondary for this reason, and not the first reason at all. He doesn't dominate anyone but the worst characters, but he can go into basically every match with the same gameplan, and it will work alright. He is nice to pull out if you are losing to something you don't fully understand yet, such as a megaman shooting all your bananas and you need a quick response before you are 2-0'd.
I think Doc makes a great secondary for a combination of these reasons, as well as being the 'what?' character too. Many people probably don't know they get kill confirmed by dthrow fair, so he has a certain pocket value for these MUs. (When this confirm kills, it kills early. It also works on light fastfallers, i.e. the most important characters.)
The downside is, people don't have to know Doc in and out to know that his weakness can be exploited, since exploiting those with lack of mobility isn't a new concept. However, with dynamic stage picking at tournaments, you can sometimes 'corner' someone into a game at Dreamland or TaC and not have to worry as much about being camped, before you pull out the secondary.
Either way, I agree the meta is now leaning towards the value of having a few characters under your belt. With new top tiers with new top tier traits and weaknesses, certain properties have usage now! Mewtwo was buffed and never had much problem with Shiek, sure, but he is finding a strong usage case with a powerful throw game against a character that can counter striking with precision. Speaking of which, his instant death isn't so damning against bayo, she does it to anyone anyways so Mewtwo doesn't exactly get a strength in the MU, but his 'weakness' is put on an almost level field ( I say almost because Witch Time and Dair / Bair will still kill him early)
So? Well, I find it exciting. I always vacillate between characters, never being able to decide a main. Maybe I can just play 4 or 5 characters with decent proficiency, and have a powerful tool at the mid-low level! That is cool to me.
Also, seeing someone like Larry pull out a DK is also very cool to me. Smash 4 is in a great place, and the soft ban on Bayo has a part in this.