Given the lack of of any evidence, I assert that most
of the percentage
of people who a) notice a problem, b) care, c) know how to fix it;
probably deserve to be using the resources anyway. Besides anyone who
doesn't deserve but still fixes the problem will likely be able to, and
want to, circumvent other measures.
It's the last point that's the kicker. I don't have any
evidence, either, nor do I know precisely what problem is
attempting to be solved, here. "Spamvertisers" have been
mentioned. The impression I get is that when it comes to
spamming, the vast majority of the damage is caused by a small
minority of operators who are extremely motivated and have the
resources to hire arbitrarily talented programmers.
Therefore, an approach like this might block a large number of
the nasties, but a small percentage of the total damage.
So, in the end, if the spam problem ends up being more or less
exactly as bad as it was before, then all of this is actually
a net loss. Not only are the spammers unimpeded, but the
collateral damage is still exacted: the unknown numbers of
innocent bystanders (who, for whatever reason, don't have
User-Agent supplied for them and aren't in a position to complain
about or fix it) remain excluded. Furthermore, once we've
taught/forced the canny spammers to undetectably spoof the
User-Agent string, that string becomes that much more useless,
not only to us, but to everyone else on the net, too.