Sure, I ask this question about lots of stuff in my life, hobby or otherwise. Sometimes it's about money and sometimes it's about clutter or space depending on the object. Whether you consciously asked yourself the question or not, everyone who buys a pinball machine has answered that question that yes it will bring them $X of enjoyment, because otherwise they wouldn't have made the purchase. It's a perfectly normal thing to continue asking yourself that question because maybe later the answer is no, so you sell it.

I don't understand the answers that asking yourself this about pinball is somehow ruining the hobby. If that's the case I can't imagine your angst when spring cleaning time comes around and millions of people are asking this question about millions of different things. Oh the horrors of healthy self reflection.