Bromptons are quite small and hardly take up storage space-even in the cramped quarters I keep mine in. Besides, have you considered the possibility of back-up in case one of your main bike(s) are at the bike shop or otherwise unavailable? Or you decide to ride it in an area where you cannot lock any bike up safetly and a folder like that would fit your needs then? Take careful stock of why you and others buy these bikes in the first place. I don't think it is because of it's fantastic riding/handleing qualities or brakes.

If after these considerations are dealt with, then sell. You should not have regrets at the wrong time!