I am all for a second toilet and I like the idea of adding one I wish we could add one here.
We have a shower over bath, think old style old house with crappy shower curtain arrangement. I agree you can do lots these days and the idea of it wouldn't phase me, and if it gave you extra room then it's a great idea.
The only main problem we have is that DH nan can't stay as she can't manage the shower bath, so would only put off older potential renters.