Towershields grant a +4 to AC and when you take the fully defensive actions, they grant total cover accept against spells (it does not block line of sight). It weights 45 lbs. and it gives an armour check penalty of -10. It also limits dex bonus to AC to +2. You also cannot shield bash with it or do anything else with the shield hand. Finally, only pure fighters have proficiency with this shield, all other classes need to spend a feat on it. The feat has the requirement of having normal shield proficiency.
In short, you will not see any wizard or rogue walking around with this shield. It has become a more valuable option for fighters and clerics. Clerics really have to spend that feat, because a -10 to attack and all skill checks hurt. Unlike light shields though, the cleric will need to put away his weapon to cast any spell. Barbarians might opt for it as well, but those characters more often then not prefer 2 handed weapons, especially with the boost of powerattack. Rangers run the risk of getting into medium load due to this shield and they will focus on two-weapon style or archery (which requires two hands as well).
I am sorry, but I don't think the tower shield is really that powerful.