It's a risk, and it would be great if they worked on more sites, but "utterly useless" is too brash in my opinion - there are still a lot of sites that do accept them, and each one is one account fewer that can be linked to your others. I don't think I've had problems receiving concert or cinema tickets, for example - the one-time uses for which they are most beneficial. I just checked, and I currently have 385 email masks used. Certainly worth the €1 a month.

(Disclosure: I used to be on the Firefox Relay team.)