If you don't want to bother with parrying and / or play with friends and want to be the tank then it's okeyish. If you use spear with it, you can attack and go back to block in a half sec. But you can do it with round shields as well so..

Heavy movement penalty and easy parry mechanic makes it weak compared to round shields. Succesful parry will block more damage than tower shield of the same tier.

I use them simply because they looks cool.

EDIT: if you play pvp then tower shields can be better choice since it's hard to time parry against other players. But most would probably still go with round one due to movement penalty which is crucial in pvp.