I'll throw in my tuppence worth.
I agree an auxilliary engine on anything but bigger boats can be a pain. They rattle (unless you strap 'em down), take up space and add weight, and chances are will rarely be used.
On the other side, its very, very reassuring to know you have some means of propulsion should things go wrong. Yes, there's VHF... but even that isn't a guarantee if you're in a remote location or there are no other boats out (I'm thinking west coast here).
I've fitted auxilliary engines to a couple of my ribs over the years, first a Tohatsu 3.5 on a Zodiac Pro 7-Man, and now a Mariner 4hp on my Ribcraft 4.8m. Frankly I wouldn't be without an auxilliary now.
As has been mentioned before, even new engines or reliable engines can fail. in addition - I can think of numerous situations (including ones I've witnessed or had near misses with) which can disable your main engine, regardless of condition or reliability, including:
1/ Lobster pot marker buoys (wraps around prop and strips rubber bush)
2/ Hit flotsam including partially submerged logs, fishing net, etc
3/ Water intake blocked leading to overheating (plastic bag, seaweed, etc)
4/ Contaminated fuel and/or damaged primer bulb
5/ Battery drain/starter motor fails
6/ Manual start engine - starter rope breaks (don't ask me how I know)
Long story short - if you boat on your own, in remote locations, and conditions dictate that rescue and or tow is a bit of a lottery, then go ahead a fit an auxilliary.