I measure and my registration rates have not suffered. I don't block the content to those who don't turn off their blocker. What I block is the ability for the blocker to by-pass the whitelist notice. Measure, measure, measure. And A/B test. And remember that measuring is not simply relying on Google analytics -- since this is blocked by many ad blockers it gives inaccurate data. I use beacons as well as a raw server log analyzer.