Of course you can't field a team full of them, it's more or less a one-shot weapon. Motivation greatly drops once this is done it is impossible to maintain a good form if you use him too often, but for a while, or that ONE game, even a 1* is comparable to a fair-conditioned 8* player.

And a backup GK improves form, I always make sure my team to have 2 GK.

There is no "waste slot" for a low star young player, as they are profitable to train anyway.