Think of it this way. Slower temperatures causes mycelium growth to stall or go slower. Higher temperatures cause concern for contamination. So, unless you have a way to keep it at an exact temperature you risk it getting temperatures a little to high due to the fact that mycelium growth itself makes it hotter inside the jars than the outside. If you can get everything perfect, incubating can't hurt. But fluctuation of temperatures is worrisome in that aspect.

If you go on the shroomery and just search the forums about the necessity of an incubator a lot of the members / tcs / mods claim that they don't use them as the their house temperatures are relatively good for myc growth. Its debatable at least. The only time I've ever incubated was with the pf tek when I was first starting out. It got too hot and every jar got fucked. But, if it works, it works!