The shower is usually plumbed on the same wall as the toilet because it is easier for the plumber that way, not because it is any less expensive. I had my shower plumbing put on the end opposite of the toilet, and it makes a world of difference. No more reaching around the toilet to turn the water on and off. With a tub, you just turn on the water through the tub spout to get it warm, then after you get in you can divert it to the spray. If you have young children to bathe, this setup is soooo much more convenient.