I always keep my old pj's - I have plenty to paint and would rather keep the history/ development and keep moving forwards. I'll strip anything I buy painted that I want to paint myself (I don't collect other people's development

) and anything I want to sell that someone else painted, but my pj's? No, I'd feel like I'd wasted my time if I stripped them.
The only exception I guess would be if I couldn't replace an old model and wanted it painted in keeping with my current stuff, but generally I'd hit Ebay to buy another model if that came up.