First and foremost, if they're talking behind your back, you probably need to change your crowd!
When I introduce friends to my collection, it's usually the volume of machines looking peculiar, (after all, seems like most of us have more than 10). I can understand it appears like overkill since they don't understand the huge difference in theme and play variety, and of course, the amount of space needed. Never-the-less, once they tried playing, as I stated before, my parties went from closing up at 1-1:30 to around 4-5 in the morning. Couldn't get rid of 'em.
The car and boat comparisons do not work with me. Although cars are a household item. I see very few with more than 2-3. Boats are luxury items obviously, but again, few have more than 1. I think 30 pins in the basement sounds a bit more unique than having a boat in the driveway.
Remember, a hobby is your source of enjoyment. ROI is just a by-product. As long as you had fun, who cares?