I completely agree with this. If you're going to post your fill to Tumblr or your AO3 or whatever, you're going to de-anon anyway, so I don't see the point in insisting on anonymity for fills.

However, I like the idea of prompting being anonymous, because it prevents possible favouritism (like more well-known/popular fans getting their prompts filled) and discourages wank, which tends to happen when people put a name to a certain style of prompting.