I haven't ridden a single-disc Monster, so I've got no FHE.
Personal opinion, my S4 has more than enough braking power with the twin discs. Maybe if I was riding really hard with a passenger, on a steep switchback road like I've seen pictures of in the Alps and such, I might wish for more. Or if I were racing. I'm not likely to do either of those, so I can't foresee ever running out of brakes.
Lots of folks here are happy with the braking on their single-disc bikes. You can upgrade a single disc with better pads, or a cast-iron disc, or a better caliper, or all of the above if you like. It would still be lighter and less complicated than the twin setup. If you find you really need or want the twin setup, that upgrade is pretty straightforward, several folks here have done it.
i thought the braking on my 02 m620 was decent or good enough compared to the few other bikes i've ridden. then first time i hit the brakes on the 749 i almost flew over the front of the bike. looking into upgrades for the monster now.