Since people have been using anecdotes to debunk some anti-customs arguments, I'll share some of my experiences that counter those anecdotes.
The Logistics of Setting up Customs for a Smaller Scene
Not all regions have someone who has a Powersave and can easily unlock all customs on their 3DS. In my city, nobody has a fully unlocked 3DS or a Powersave and the people who own Smash 3DS are not supportive of customs. As such, we're forced to fully unlock customs on our Wii Us. Even with the turbo controller method, it takes more than a week to get a full unlock. The community also has a fairly limited supply of turbo controllers available, so we can't all unlock customs simultaneously. In addition, as someone who owns a turbo controller, there are people in my scene that bring set-ups but that I do not know very well, and I wouldn't be comfortable lending my controller to those people. The result of all these effects is that getting customs set up in my community has been a very long and tedious process that is still unfinished. If we choose to run a tournament, we can choose to run on either 6~ set-ups with customs on or 12+ set-ups with customs off. It's simply far easier and more efficient to run a customs off tournament.
People Drowning in Pools at EVO due to Customs
I won't go into as much detail as Amazing Ampharos regarding my own EVO pool as most of it was uneventful. However, I played two sets where customs played a huge factor, and was able to advance from my pool because of customs. I mainly played Mii Brawler because I thought some regions and players were not prepared for this character. The other characters I play are ROB, who is likely my best character even with customs on, and Rosalina.
My first set was versus 8bitman in winner's semis of my pool, who played ROB (1231 or 1131 I believe). We gentleman to Smashville because I'm a scumbag who takes advantage of player laziness to get one of Mii Brawler's best stages game 1. I win this game because I get a platform enhanced Helicopter Kick combo at around 40%. He takes a moment to recompose himself, wins a sort of close game 2 (he had a huge lead but I could've killed him off 1 grab because it was Mii Brawler vs ROB), before taking a hugely dominant game 3. He ended up winning so it wasn't the biggest deal; however, I definitely would not have taken a game against him without Mii Brawler. As a ROB player, I could recognize his ROB was far better than mine and my Rosalina is pretty garbage (while he has taken sets from Xaltis' Rosalina).
The next set was in loser's finals of the pool against Mav, who played a custom Mario (High Pressure Fludd and Gust Cape I think) and custom Fox (Twisting Fox, the wolf laser blaster, and I forget the rest). I 100% won this set because of Mii Brawler. I watched him play 8bitman, and I feel like he would've easily 2-0'd my ROB. However, he DI'd Brawler's dthrow outward and died multiple times at 40-60% from Helicopter Kick combos. This is a player who was prepared for customs, used his own customs well, knew about the Mii Brawler stuff, but lost to a worse player because he wasn't experienced in the Mii Brawler matchup. I do not feel like I would have made it out of my pool if customs were off.