Chain tensioners are used for retro-fitting IHG onto vertical dropouts OR by lazy manufacturers who use a bog standard hybrid frame for their IHG model.
You shouldn't buy a new bike with one.

You wont get a racing-speed wheel swap with an IHG but its no big deal. There is a trick to unhooking Shimano cables from the hub, just a small twist and the nut pops out of the cassette arm.
I like my Alfine hub and wouldnt go back to derailleurs for everyday city riding.