You're all answering your own questions really - I think the point may be that not every airgunner has £400 - £500 worth of scope atop their combo.
Personally, two of my rifles have IR, and I've used it to good effect in low light, just has already been said. I have Hawke scopes on all my guns, (only one of them cost more than £100 - and that's the only one that doesn't have IR, strangely) and they all do the business for me. I wouldn't let IR be a selling point, but if you've got it, in my opinion, it can be useful.