When you put it that way I agree with you
Maybe I was thinking of newer players. Lets say e.g. the most you know about the sicilian is 1...c5 is the sicilian. Well you can at least practice some of the basic variations and get an idea for the type of play that follows.
Ok maybe not an ideal training method, but it doesn't seem so bad.
I guess the biggest problem for more experienced players is on most sites people play trash openings in blitz. Lets say I wanted to prep some main line ruy whose position starts 15 moves deep... well I'll probably never see it in blitz unless I'm playing GMs or something.
"Blitz is useful I think. Both to test openings and little patterns you can pick up that help speed up your analysis in long games. Even simple stuff like a bishop on c2 covers all of a c5 knight's forward squares or maybe it's a mate-attacking pattern you hadn't seen."
This has not been my experience. Even the tactics I lose to I've seen before but couldn't see them at the time, and whether I would miss it in a standard game is very hard to determine because it's such a different situation. It's just very hard to learn new things when your only coherent way of playing in the time limit is relying on ideas you have already used exclusively. I learn something a lot better if it stared at me in the face for a long time (in standard) but I still couldn't find it -- that would demonstrate a clear defect, but I would be able to correct it by recalling my thought process.
The part about openings has some point to it, except that how an opponent reacts to your opening in blitz is not necessarily a good indicator of how they would in standard. It's reasonable that an opening idea that works great in blitz would work poorly in standard, if it's just a matter of finding a few specific refutations that would be too time consuming in blitz. In my game with Infiniteflash for example I would be very comfortable carefully meeting white's ideas, but in blitz it was a bit foolish to go into it because there is no way I could find a deep defense in that situation (although it did make the game more exciting )
But truth be told, I'd be begging for him to play that line against me in a standard game where I got the two extra pawns.