I wouldn't say that. Lamarck's theory wasn't pulled out of his ass; it fit the facts available to him, just like the phlogiston theory did. It happened to be wrong, but it wasn't because Lamarck was stupid.

Incidentally, leeches actually do have a place in modern medicine: after reconstructive surgery, they can be used to draw blood out of the affected area in a gentle, controlled fashion, reducing swelling and internal clotting and letting the several blood vessels heal much more reliably. This has nothing to do with the ancient use of leeches, though.