I'm just going to say that if buffering was allowed in tournament play I would just set my buffer to whatever it was to make it so that shield grabbing fox and falco out of every shine attempt was mind numbingly easy. That said, I'm not sure if it would be enough to counter the fact that it would be much easier to do shine shenanigans, a technical barrier that has, in large part, prevented Fox from being the theoretical ultra character that he could be.
My point with the above paragraph is that there are A LOT of things that would upset the balance of the game, from subtle to not so subtle, if buffering was allowed for competitive play. It's much easier to balance the game around no buffering, and just deal with it.
Also, PM and smash in general has technically demanding characters and characters that aren't very demanding at all, and they come in different varieites. The tech skill for playing Samus is very different than Fox/Falco, and it's even more crazy with Ice Climbers, and then there is CF, which is less about crazy tricks and just the mastery of textbook technical skill. The awesome thing about melee is that if you liked technical stuff, there were several viable characters that offered different flavors of immense technical prowess, and if not you still had a large roster of viable characters that weren't as demanding.
It seems to me by adding buffer in, it removes a lot of tech skill across the board. The super fast fox/falco inputs become easy, the precision timing of Samus gets toned down (although it would more than likely **** it up completely, which is even worse imo), and the timing tricks and desynching with IC's gets jacked up as well. It would even make the less demanding characters even less demanding... which is just... well not good game design.
I'm not going to go into details over what makes something "competitive" or not, and honestly a game is usually as competitive as it has competitors. Even in the most simple of games there will be refinements to be made to improve, and human error will always be prevalent. What I will say is this, smash is great because it's easy to pick up, but hard to master. That holds true with PM. The technical barrier in PM is less than melee, a game that has lasted since 2002... that should say something about it's design. Something about it connected with people, and it has lasting appeal. The jury is still out as to whether or not Brawl could last that long or not, but honestly PM is sorta the best of both worlds, and an excellent bridge between the two games.
All of the above applies to l-cancelling too. It's clearly a mechanic that works, and it is not unlike technical aspects in other fighters (like linking combos). It's not a hard mechanic, but it does demand that you have to spend some time working at it. In reality though, by comparison, it is far less demanding than most other fighting games and their more arbitrary tech barriers. Also, the fact that you can mess with someone's l-canceling by mastering the very, very subtle mechanics of shielding. That may seem small, but that adds A LOT of depth, and a lot of stuff in high level play. That is stuff you didn't even see until many many years after melee's release, yet pretty much every player learned about angling their shields, and using the light shield after playing for several hours.