If you regularly ride on the beach, you can worry about rust in 10 or 20 years. Otherwise, it's a pretty baseless argument.

Think about why you have several bikes--can you do everything with one? I often daydream about going down to one, but usually I have a bike set up with fenders and racks, one with a fixed gear, a mountain bike, one undergoing some project, etc etc. Yes, I could do all that with one bike, but it would mean spending a lot of time in the garage shifting it from mode to mode.

Eric
currently 3 steel and 1 scandium, but I had a steel MTB in the past as well