I HATE save-the-date cards for anything. I think it's another way for the industry to make money and sell you stuff you don't need. The way to invite people is to send an invitation, period.
For a shower, the hostess needs to check informally with key participants (like the guest of honor, her mother, etc.) and choose a date. Then send invitations to guests.
For weddings, I don't see why a family can't casually inform their immediate relatives and key friends ("we're so excited that the kids will be getting married on June 12th") while the bride/groom invite their wedding party. They can start to inquire about hotels for out-of-town guests and so on, but the real invitation is...THE INVITATION!
I agree that save-the-date cards are designed to make you mark your calendar before the hostess gets her act together to send invitations. But it's twice the work, really. For weddings, usually the save-the-date card also includes the wedding website to let you know what you should be doing about gifts. I find that offensive. None of this appears in any etiquette books of any standing.