Under the Default AI attributes for fullbacks, I've had found that the position is highly over-rated. So what are the important attributes for a "thankless" position?

I have found some-what success in moving FB's to the interior guard positions since their default pass-blocking/run-blocking attribute ratings are similar, even though FB's are smaller and lighter but faster than the Offensive linemen. Their better-than-average OL speed helps getting to the second layer and blocking LB's.

Just curious on other players thoughts

I have had success converting them to H-backs too. Because they tend to be very useful on special teams, the position is not really "thankless," but neither is it a legitimate offensive position.

I like the FB myself. Ive found the fullback to be important for the inside run game. He has to have the accel to hit the hole in time, speed to get to the LB and strength to block them. They also help with pass blocking especially if your Oline is struggling. If he can catch and run routes you can get mismatches by sneaking him out with the swing pass every once in a while. You only need one of those in a game to make a difference sometimes. So to me he's a worthy investment that you usually can get fairly cheap and use on special teams as well.

And like slowtospeak said, some of them are more suited at RB if they have the talent and ball carrying skills.