I think they're a fun little surprise when they happen, but I wouldn't pay extra for them. Except for our DCL cruise, we've never had them every night of a Disney stay, so we don't expect them.
Leaving a nice tip for the maids the first morning MAY increase your chances of getting a towel animal. I know on cruises every time we leave a small tip on a towel animal, the next day we get an even bigger and better towel animal.