As a free action, you can change the tower shield's facing once per round. A normal shield has no facing, and protects you from attacks from any direction. A tower shield also provides cover to the defender as well as the attacker. At 45 pounds, you can't walk around holding one out. (try it at home, find a large nag of dogfood and hold it out in one hand...you won't be doing it long).
They get bought but are eventually sold or not replaced, large wooden shields (Roman shields, Scutum) are what is used instead, eventually, because they float (don't want to lose them in a water accident)...