Subjects as specifiers

While reading Syntactic Theory by Sag, Waslow, & Bender, I once again ran across the idea (mentioned previously here), that the subject of a clauses (where Clause = Subj + VP) is analogous to the Specifier^† of an NP (where NP = Specifier + NOM). Here, however, the example given in (34) on p. 64 is unambiguously comparing a clause with an NP:

1. We created a monster
2. our creation of a monster

They use this idea in their constraints-based grammar to reduce the number of rules needed. Thus instead of defining NP one way and Clause another (they use S, but I'll stick with Clause), they can use one feature specification to capture both as in figure 1^‡:

Figure 1. A feature structure representing either Clause or NP.

(At this point, things get very technical.)