Isn't there a difference between using ellipsis to bring a construction into the rules, as Phil so nicely did, and adding ellipsed constructions for no reason? Once again my problem here is that grammarians love to explore exceptions to the norm. As a result, they end up presenting students, who do not yet understand the norms, with a bewildering array of exceptions. The result is that most students, and many teachers, hate grammar.

Ed,
   I wouldn't disagree with your decision at all. It's hard sometimes to
know how complex an answer to give when questions come up in class.
Simply saying "this is how I understand it" is a reasonable call.
    In a sense, pretty much all adjectives and adverbs could be subject to
this kind of ellipsis. We can overdo it. (He jumped as quickly as he
could. He made the circle as round as he could. You could assume
"quickly in speed" or "round in shape".) "As long as he could" is
clearly adverbial without the "for". If we turn it into a noun phrase
with "for" in front by positing an implied noun, you are taking an
unusual step to salvage the purity of a rule, the kind of structure
that combines with a preposition. It may be just as simple and elegant
to call it an exception. At any rate, "for" adds duration or makes
duration explicit. Herb's point, that it adds aspect (duration) to the
verb phrase, seems reasonable as well, especially because it directs
attention to the kind of choice a writer might make for clarity.    >
    It certainly makes sense not to get too bogged down with students.
There's more than one way to simplify.

Craig