Its just too much time to invest for the casual mapper to create a full custom map, especially if all they have to start with is a limited knowledge of UED.
Those who started mapping in the early days of UT started at the right time. As 3d gfx were developing and the scope of what you could produce was more limited. As new technologies emerge they could incorporate them into their skill set. The scope of what goes into a map these days is enormous. I wonder if the professional level designers who have commented on this thread create all their own content for the maps they work on or if they work on their maps as part of a team with certain individuals focusing on creating custom assets.
For someone to start mapping now to get to a high level of competency to create a pro-ish map with a good amount of custom assets, they will need to learn UED, 3dmax, ZBrush and Photoshop and all the while developing that eye for design AND gain that sense of what makes a good playable level.
Personally i can use all the above, i do have a design background (not games) and i still wouldn't make a full custom map because 1- i'm a casual mapper with zero desire to get into the industry and 2 - it would be far too time consuming, i would have to give up every other freetime project i had on for a number of months.
I think if the community demands that all their mappers produce fully custom maps then they are going to see a lot less emerging talent coming through as compared to ye olde 99/2k4 days as the demands are far too great, especially for the casual, time deficient mapper.
Mappers today for UT need to be given a LOT more custom assets to play with, otherwise by the time they've finally created their own unique map the rest of us will be playing UT4. I think one solution i would like to see is if epic released a lot more custom assets and introduce a few more new styles, maybe to compliment some new teams for the game. I think another 5 new styles of custom meshes for mappers to draw upon would really help.