I did that. You can't very well :USE MK into CL-USER on Allegro, because Allegro's CL-USER :USEs their :EXCL package, and their :EXCL package contains their own DEFSYSTEM. So using both makes a big mess.
When I looked into this, I found that it was really painless, for the few places where the MK-DEFSYSTEM stuff *was* used, to just turn FOO into MK:FOO.
At the cost of a few extra characters, we avoid a gross effect on the CL-USER package. Seemed like a good tradeoff to me.
BTW, I'm pretty sure that this was earlier discussed on the mcclim-devel list, since I have only recently acquired commit permission. So I managed to persuade the community at large that this would be at least an OK Thing To Do.
Best, R