The prefix naming convention really isnt for the end user, because as has been said, these names are normally hidden under the covers. Most of the visibility the name gets now is due to everyone pumping these things out so fast, before the SDK, etc comes out. There will come a day when 'map' wont be the preferred invocation to run these things, and people will be delivering "map packs" and "bonus maps" instead.
Likewise, I think the arguments about there not being differing playstyles in Portal, so no need for different naming conventions. . is bogus. The game is in its friggin infancy right now, and in 2 years there is no telling what directions this can go once people get their hands on it and start working with it. For instance, I can forsee one set of people being happy with strictly puzzles, etc. . . .and another set of people liking maps that makes heavy use of the portal gun in combat/tactics in combination with the gravity gun, etc. (yes, that could be a mod. . .but it doesnt have to be).
But really, none of that matters greatly to me, because I know that no matter whatever anyone else does, I myself WILL follow a convention (and I'll gladly take whatever existing convention comes about). It costs little and may save ME a lot of trouble later, even if I dont see how today. If you've never been burned by not having a standard, then you probably dont understand that kind of thinking. If you have been burned, or had to impose organization/convention on something after its all been done without a standard. . .then you wouldnt even think of NOT doing it, and this conversation would never happen. 