Yes, it wouldn't make any sense with a 4 row keyboard. But with a 3 row keyboard the alternative would be much worse: putting the space to the center would break the qwerty layouting, pushing letters to strange positions, and that would be much worse.

A three row keyboard layout with space bar on the right hand side is for instance in the Nokia N97, or it was (basically) also on the N800 full screen keyboard. Those layouts have been certainly usability tested, and it isn't causing any major problems to users in tests. I'm personally a lefty, and I don't have problems with the N97 layout and space bar position.