Conrad wrote:
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.