I see the potential for overuse point, but I think it's all good. Trying to define usefulness deserving of thanks for a whole community of online users from around the world is a lot to expect. Between parents, coaches, and gymnasts there are varying levels of technical knowledge, so only thanking the most educated would be exclusive to the point of alienating others. Only being able to thank answers to questions leaves out thanks for insight or a new perspective, or for someone just looking for encouragement.
Personally I use the thank option more often to see what other people are thinking. If I see a bunch of multi-colored names beneath a post I take it as a sign I need to read the post. The thanks beneath a DD accomplishment thread post is warranted I think. We all love to brag about our kids, and gymnastics is a hugely demanding sport on kids and the wallet! When someone has a reason to share, the thanks keeps the happy train going on for awhile longer, and a positive train of thought is something to be thankful for!