My thoughts exactly. I mean how often is it going to come up at low levels where a ranger crosses terrain and needs to make a check before the long rest.I don't really understand why it should be terrain dependant at all. Especially if you can just swap it out anyway.
And at high levels wtih teleports and the like.....well is there REALLY a problem with a ranger just having the ability always working at that point?
I think you just give them this bonus straight up. Anytime they are in a natural terrain, they get the bonus. Simple and easy, just be done with it.