The problem with allowing one titlepack is that you have to then allow all titlepacks, otherwise things feel unfair. This is especially true for titlepacks such as the ESL titlepack, where there are a lot of awesome new items for Stadium and that's it. Canyon and Valley are now left out of the fun because they don't get the same variety unless you allow all custom titlepacks. At that point, however, we might see every course on its own titlepack, and some people don't want to (or can't) download more titlepacks. Because of this, they are essentially barred from judging.
There have been MTCs in the past where custom titlepacks were allowed, whether all titlepacks or just specific ones. I believe they were fine for the most part, but problems have come up before (such as certain players being unable to even use a titlepack because the menu doesn't load). I'd like to be able to use custom titlepacks, but I can see why they aren't allowed.