That is one reason, in a sense. It is also the fact that people will hack into and edit the base coding of a game in an attempt to change it because they don't like it. I'd ordinarily have no issue with that what-so-ever, but Brawl is still new. After 3-4 years I can understand wanting to change something old and reinvent it to bring some nostalgia back to the game, but Brawl wasn't even close to that point when the Brawl+ project first started.
A scary thought arises from this: what if this can be done with any game? Editing it to the editor's liking? We aren't back in the days where guides for every game existed and tutorials were everywhere, including in the game itself. We had to learn by playing and intuition. Trial and Error was a necessity. Memorisation was required. It was hard unless it was easy. If it was easy, it wasn't worth your time. Now, if you don't like a level, edit the code and change it to make it a cake walk. You don't have the coding experience to do it? Someone already did and you simply have to download the patch.
There is also the point of stubborness to be made. Rather then try to adapt Brawl to the point where it is faster then when it first came and do as much as we can in those 60 frames a second that we are given, people are compelled to just simply change the new to try to fit the old. We all know Brawl has problems. It is undeniable. But what is wrong with simply trying to fix the broken by working around it instead of attempting to cut it out completely or add to it so much that it no longer seems broken.
Yet another point to be made is the split in the community, which is never a good thing. With a split the Metagame of every single character suffers on both ends. Instead of having 2000 advanced players focusing on... let's say Ganondorf... we will have far less. This increases the amount of time it takes to develop the character in question. This could lead to us missing the game-breaking AT that works with Ganon before Brawl's life cycle is over. The one that would make Ganon a god. What's worse is that I've seen massive communities crumble and die for lesser things than a simple hack.
Aside from splitting the community, it lessens tournament sizes quite nicely. Look above you. Look at the past posts. People have been saying Brawl+ > Brawl. I'd like to hear the logic, but I doubt I'll get anything more than an equivalent to "it's fun." Brawl+ takes players from the Brawl community and lessens the number of people that go to Brawl tournaments. Good luck to us if we want to get a major sponsor like MLG. Money is power. If we don't have enough attendance to make them money, we drop from power as a major competitive game. I also dread what will happen if Brawl+ starts to get tournaments going. It is doubtful because of what is required to simply play Brawl+, but I digress.
Now, the mere fact that I'm on this board shows that I don't overly care about original designer's intent. We all know Brawl wasn't MEANT to be a competitive game. I doubt Melee was, either. I wasn't playing Melee when it first came out. But I do respect original designer's intent enough to recognise that a game is more then a game to that designer. The designer put years into the game trying to make it the best they can just to have it edited by someone who didn't like an aspect of it.
I'm designing a hack of Super Mario RPG: Legend of the 7 Stars right now. It will take me months to perfect and complete. I can start true design of it when V1.1 of the editor is released. Once it is done, I don't want someone who is having difficulty on one part of the game to edit that part out. I want them to seek advice or message me with their concerns on difficulty. It is a respect thing.
My final point is what people say to people like me that don't play Brawl+ for whatever our reasons are. I asked previously if there are any Brawl or Melee tournaments going on around Southern Idaho. I mention that I don't want to be told of Brawl+ tournaments and I get a few messages saying Brawl+ is better. I get no answers for my question. I don't even get pointed in a direction that could lead me to the answers I am seeking on my own. I'm sure if I made a new account and did the same thing in a week that I did recently I'd get more answers like I already have gotten: none that are conclusive. It is causing people looking for answers to be shot down by people who play a hacked version of the game you have questions on or about.
It all comes down to this: Brawl+ is fun for you. Ok. I respect that. What I don't respect is the overall harm it is doing.
Now, before anyone responds to me, because at least one person will, (as is the nature of a message board with a massive post) I would like you to read each and every word of the above. I want you to think about it. Try to remove any bias from yourself. I tried, and I failed to a degree. I can only hope it was a small degree. It is to be expected, however, since that is how opinion goes. My opinions of how the community and Metagame grow, and also what respect is and how it should be treated, are all in this.
The original question wanted me to take a game-design point of view. I did near the end. A topic like this, however, can not be looked at purely from a game-design point of view. I hope you understand.