I doubt adding a new derivative costs the KEIL folks over the cost of one copy of their software. They don't support the additional hardware in any way. They don't support the peripherals in any way, since they don't simulate them.
The problem is that it is not a matter of adding a new derivative, it is a matter of adding 100s? of new derivatives every year.

I think I have to agree with Jez' initial assessment, that the 2KB-limited version is inadequate for evaluation of the features that ultimately make the difference
Absolutely, however, the 'eval' is not for evaluation :). With the exception of Tasking I have NEVER had a problem borrowing a full tool from anyone for an actual evaluation. However, when 'evaluation', as often is the case, is a thinly veiled attempt to acquire tools for free, the limit is quite reasonable. When the makers name a limited version an 'evaluation version' there can be no argumenif the manufacturer has ANY suspicion as to your purpose of acquiring the software.

I can not count the number of bootlegs I have seen in my career and I DO understand the manufacturers reluctance to let the cat out of the box. On one occasion, I did not get a consulting contract simply because I refuse to work with bootlegged tools (not a Keil bootleg).

Erik