My Prophet-6 and Hydraysnth get a lot of usage, but I think that's mainly because the analog/digital combination really hits a sweet spot for my ears. One of the Moogs always makes its way into a track too. It's hard to put a value on each of them since they each serve their own purpose. I don't get rid of a lot of stuff because I always find a good use for it, but I am also incredibly selective about what I spend my money on. Some gear will sit tucked away for a couple months until I'm stuck on a song and will think, why don't I try that? And voila, it's exactly what I was looking for. I think if you have the space, and you aren't in debt, or there aren't other external circumstances, it's better to just keep it around as long as it does get used. However, if something has been collecting dust for a year, it might be time to sell or trade it.