It's not venting frustration, it's telling visitors there is no free lunch.
Letting them know the site uses ads to make money, you want the content you see the ads.
The "you need to unblock" page could contain instruction for Norton Firewall, etc.
Yes, but what happens when the next version of IE blocks all ads? Firefox already has this capability so soon your revenue will dry up when everyone and everything blocks all ads by default.
Spiders would be allowed to visit the site, that's not a problem.
If you don't start fighting those blocking technologies now there may not be any revenue left when you decide it's time to fight this trend. Imagine if one large software release, let's say MS IE7, blocks all Google Ads by default when it launches. Of all companies, MS might benefit in the long term from blocking these "insecure ad technologies" and then put out some new offering of their own.
Not a sky is falling mentality, but the ad blocking trends are becoming increasingly alarming and if publishers don't do something proactive to educate our visitors of what keeps us in business the ad networks could collapse.
That would leave the few of us still standing either running sites for free or adopting subscription models and we know how well THAT works.