They are free to enter. Plus one point. They offer a guaranteed opportunity to be a part of the exhibit. No jury process, no fee. That's two points. They usually ask for something small, 5" x 5" or so. Not a large commitment. Three points. It seems like a win/win for both gallery and artist, but recently the trend has spread like wildfire and it is now no longer possible to donate to all of them.
Here are some things I am now considering when I find such an opportunity. Will the gallery release the name of the buyer to you. If not, you may loose a potential life long buyer. Will the gallery return the work to you if it is unsold? I have had some experiences where the gallery does not even inform you whether or not your piece was sold. I have always been able to get a satisfactory answer, but it takes some persistence. Now your small gift has become a nuisance and a major time waster. Blah! Is it local? If you cannot attend the opening, you are unlikely to gain any interesting contacts and likely will not meet the gallery director, curator, or worse your fellow artists.
My conclusion. Despite all the shortcomings, if I think the non-profit has a good mission and will benefit from the donation, I will usually go for it. (assuming I have the time) If you do, don't expect much in return other than the satisfaction of helping out a non-profit at a time when funds are short. But hey, the satisfaction of felling good about yourself is not a bad return on a small investment of time. At this point in my career, I find it easier to donate time than cash.
In fairness, I am a young artist with few gallery deadlines to meet. For more established artists, I doubt they would find the time worth the while. Still can't decide whether or not to donate? Check out Joanne Mattera's art blog. I am sure her Marketing Mondays column has addressed the issue at some point. Good luck.