To echo your sentiment, It's a bit like measuring writers not by the books they have written, but how long they've sat in front of a keyboard.
Anecdotally, Not only is time a a poor measure of ability, those who use it as such demonstrate a lack of accomplishment. Compare those who write "N years of Language X" vs "Wrote A,B,C in Z using ...".
I find "I've used X for N years" to mean "I've done one year of X over and over and over" :-)
I think we're getting at the same thing. The time example is used as a comparative generalization that there is a need to focus and understand the fundamentals. The point being that given some period of time, if an average individual chooses to learn 10 languages at once, they probably wont master the fundamentals, compared to mastering those in one language. Of course there is the side case that comparing different languages and their patterns will actually be even more valuable in overall understanding, I would have to side that one first needs a deep enough understanding for that to be an advantage.
I'd rather hire someone that can give you an approximate assembly of a C function (as executed by a CPU), understand how XML documents are stored in memory for efficient querying and parsing, know how to avoid cross-domain browser security problems, have an understanding of multithreading (locks, semaphores, race conditions, etc) and an application of recursion in some other language. I wouldn't care what the knowledge is, as long as it is well rounded and demonstrates that the candidate is capable of pulling apart complex systems to gain a deep understanding in a short period of time. The ability to reason (with passion and authority) about which language or pattern is best suited to a given scenario is also vital.
Time is a terrible measure, partly because it fails to recognise that 5 years of bad/shallow/incorrect experience is worse than 2 weeks of gaining a deep understanding of a new language/pattern.