I agree with your point; however, scanning a competitors keywords tag is, as you said, just a starting point (assuming they didn't dump their entire link building campaign worth of keywords/phrases into the tag).
For example, a keywords tag for a real estate site homepage might be: content="real estate, homes for sale, houses for sale", or something along those lines. Now, those keywords might be useful for the search engines that still respect the keywords tag, but they're not any help to competition as you probably could have guess, just by looking at the site, that those were keywords they are probably trying to optimize for, anyways.
Just remember, Google's not the only search engine out there.
It'd be foolish to give away any trade secrets within those tags, anyways, so there's not much to worry about in my opinion.
