The need for a BUG in the (un)likelihood that your primary one might fail in a SD incident, seems to be a bit of stretch for me. Although I can't provide actual statistics or data, I'm wondering if being armed with two of the same weapon is overkill (pun intended). I have a few questions:
1. What is the likelihood/odds, assuming that you're not LEO, you will need to use/discharge your weapon in a SD/HD incident? Maybe once or twice in your (armed) life time? (Hopefully, never)
2. What is the likelihood/odds that your SD weapon will FTF or malfunction? I believe there is recent reliability/statistical data available on M&P's .40 cal that was conducted by BATFE, but I'm guessing something like 1 in 5,000 to 10,000 rounds.
3. Now, what is the likelihood/odds that your weapon will malfunction at the very moment you will need it for SD?
4. And if you don't like those odds, why not carry 2 or even 3 BUG's?
I'm not a statistician or mathematician, but whatever the "odds" are when combining #'s 1, 2 & 3 above, has to be pretty high. So rather than buying and carrying another similar pistol JUST IN CASE my primary malfunctions in the unlikely event that I will need to use it for SD...well, I guess I'll just take my chances with "only" one .40c on me.