Well, I don't use Forma that often. Especially not with weapons that don't really fit into my playstyle. Also it is questionable why you should put Forma in weapons that are more likely to be sold because you never play them or they are not that strong etc.
And to be honest, you can get Forma very well and you don't necessarily use them every day. Unless you are really into really minmaxing every weapon.
And yes there have been cases where you got Forma back because some weapon stats or weapons in general have changed. They didn't have to do that, but that was just service at that point.