A couple of points.

Herb,
I know that you have forgotten more about linguistics than I would ever
know.  However, I am sure that users of the language today don't have
knowledge of historical accidents.

> I'm not sure whether I want to make a theoretical argument out  > of a
historical accident.

**************
Martha's representation of the verb expansion rule is  a good example of
how a description is a theory. 

*****
  MV = T + (M) + (have + -en) + (be + -ing) + V

This rule describes your comment that the first element in the verb
string carries the tense (i.e., is the "finite" verb).

An alternate version of this formula has a different opening slot:  a
choice of T or M.

******
If (T) -- tense is separate from modals, then modals are without tense. 
That predicts a sentence like 1 is grammatical.

1)  *Bob wants to can drink English.

And, it needs to tell a story why (-s) does not go on to modals, but
does on have and be.  

Martha's alternative (T or M) does not have these problems. 

Bob Yates
Central Missouri State University

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