[plt-scheme] Why Modules?

I've been looking into units, specifically the '99 "Cool Units..." paper.  
Apologies for reopening old threads, but I did not see this answered:

In many respects (syntax bindings not included), units provide a much  
richer framework than modules - the latter cannot be assembled  
compositionally, prohibit cyclic dependencies, and do not allow multiple  
instantiations. Thus, my question is, since you make such a convincing  
case for units in the '99 paper, why did PLT find it necessary to  
introduce a new abstraction in the form of modules? Did you run into some  
intrinsic limitations of units?

This is interesting, btw, because in the so called mainstream world of  
enterprise java, people are looking outside of the language to unit-like  
frameworks like OSGi to get around the limitations shared by Java packages  
and PLT modules.

Thanks for any effort in helping me understand this.
-- 
Ittai