Warchief hit on it. The old "take one level of rogue" exploit is penalized. I won't reveal the precise calculations here, but UMD is modified by a rogue to non-rogue level comparison. If you take anything other than rogue, including prestige classes, UMD will not count as much.
As I wrote before, prestige classes have their own rewards, and people multi-class to gain particular advantages that are not available to pure-classed rogues, fighters, etc. Custom scripting in KT creates advantages for the pure-classed characters. Without the custom scripting, there's no good reason not to multi-class and take prestige classes. It becomes more about picking the right build (and we all know about certain uber combinations in NWN).
In KT, rogues are finally back in style, and I'm not talking just about level 1 rogue posers. I'm talking about pure rogues being useful again.