It occurred to me while reading the line "He was as happy as could be." that part of it is quite vague. As happy as he could be, or as anything could be? I'm not a linguist or anything, but I haven't been able to think of anything that follows a similar pattern, without following the pattern exactly (as in the title of the question).
Regardless of how well it is understood by others or how often it is used (unless this affects the answer), is a phrase like this bad grammar?
Best Answer
Well, there's no God-given list of pieces of grammar that are "bad" vs pieces of grammar that are "good"-- just people who invent arbitrary reasons for labelling things as such and then other people who decide to agree with those reasons.
So, you've invented your arbitrary reason for deciding it's "bad" grammar. You just need to decide to agree with yourself and you're done...