When people find themselves doing that (figure renting) I really think they should reconsider what their primary movie interest(s) is/are. What's the point in buying something if you're likely to sell it a few months down the line. I know circumstances can force us to sell things we would prefer not to but if the reason for selling is only so you can buy another figure.....
I don't see a problem with it. A better version of a character is coming, sell the old one. Your figure is overshadowed by other better figures or you end up not liking it as much as you thought for whatever reason, sell it. The way these things retain their value, it's easier to update the collection, so why not?
Figures that I have sold since I started collecting last year:
DX04 - I love Bruce Lee movies but my figure stood in the detolf and I hardly looked at it. Guess it just couldn't compete with Batman/Terminator/Predator
Toy Fair Batman - I used to prefer the Begins Batsuit, but I started to appreciate the TDK suit more and I didn't like the feeling that the figure was so fragile.
Two-Face - I thought the figure was great when dressed up as Harvey Dent, but the value had gone up so much it was too tempting. I really liked the figure, but in the end, he was a guy in a suit amongst Batman/Terminator/Predator
DX12 - Great figure and I miss it, but the QS was updated to be the best looking Bale Batman yet, so I had to upgrade.
The DX10 and DX11 are the two favourites in my collection right now, but if new versions come out that I like better, they will be replaced.