i've used Wyeast's nutrient ever since my first batch, so i don't have any non-nutrient batches to compare to. i've never had a stalled fermentation, my FG is always where it should be, etc. obviously this is due to many factors but i believe the nutrients are part of the equation.
the stuff is really cheap - a tube cost $3-4 and lasts me over a year. there is no downside to using it, and it can only help.
for me, the question is why not use it?!? seems like a no-brainer!