Yeah, cove, I'll admit, I've done that too, in the past. 5,000 points is better than a kick in the teeth, but is it worth the hassle of signing up for the card, then cancelling it down the track? I'm not convinced.
However, last year I signed the Mrs up for a CitiBank platinum in her name for the 60,000 points - that was enough to get us to the point which allowed us to book MEL-LHR...FCO-HKG-MEL in JASA next year, rather than YASA.
My best deal so far was a retention bonus of 80,000 points from Amex (3 for 1 for the first $40,000 on Amex Discovery) when I said I was going to close my Amex Premium, and wouldn't accept a Discovery for free - I told the CSA that I didn't want another credit card in my wallet that I wasn't using (becuse I had a 1.5 point per $ card I would use instead where Amex was accepted), so he said "would you accept it if we give you 3 for 1 for the first 40,000?" I said yes, and also got another 10,000 bonus points (2,500 first spend for each of me & mrs and 5,000 on first QF spend). So, that's around 120,000 points instead of 60,000 points, and that will allow us to waitlist for J to F MEL-LHR.
It's all a game; sometimes it's not worth playing when you take into account the cost of your time to play, but sometimes it is.