unless you encounter that emergency situation (like Saga) where you must brake hard or bite it bad, it probably won't be a big deal (just shift your weight back a bit).
However, because a single brake up front will not sufficiently cause all that weight+momentum behind it to slow evenly and as a result you may dive over your handle bars upon a sudden stop (if you have a good brake)...that would suck...I did that when I was like 10 and i still laugh about it (it is evenmore funny in slowmo memory) but dang it sucked at the time.
Seriously, I think the responsible answer is that two brakes = safer stop. Both brakes working simultaneously allow all the mass centered on the bike to slow rather than cause some kind of awkwardly embarrassing situation with you moving over and beyond your bars. On a fixed gear your legs slow the rear wheel and you don't have that same uneven momentum problem with (or without) a front brake...but on a ss the freewheel prevents that all that fun stuff.