HH gyros don't just dampen/reduce the rate of drift, they integrate it and try to correct for the total amount of drift. The error in this integration results in the error you're talking about. Rate gyros sense the drift and try to stop it, they do not correct for the total amount of drift by returning it to the original position. When you actually start flying your helicopter, you'll find that the HH gyros work extremely well in this application. Afterall, having a gyro that is trying to whip the tail around several degrees while your trying to lift off is not a desireable thing to have...