If your tool is really useful, some user programs will be copied and passed around by people unaware of their limitations and assumptions. User programmers are even less prone than professionals to document their productions, and programming is only a sideline for them. When a prolific amateur moves on, she may leave quite a vacuum. Users know their problems best. The solutions they create for themselves are rich with detailed features they wouldn't have time to specify or explain to a programmer.
Freedom begets responsibility.
That was a first in his 2 decades of customer support. In a system designed around user programmability, the system program can be orders of magnitude simpler and more reliable. Instead of maintaining megabytes of specials for their whole customer base, programmers are set free to build the general functions they are good at, while users create the specific features they care intensely about. There is a critical mass of functionality before end-user programming becomes useful. However once it is reached, tools don't need to be fancy.