The payoff time is not exponential by adding more of them either, it will take around the same 160 days for one or for five (1500 mh/s if you're counting).
This part is a very valid point. Consider the increased earnings by having more though. After that (160d) you would be looking at, say $1,500 a month instead of $150 a month by having 10 instead of 1 (all using theoretical numbers). Of course, the investment is also larger, increasing the risk.