there is no heat issues,
and it won't hurt it whatsoever.
actually, i heard it makes it even better for ventilation since it props the laptop up a bit so there's air room inbetween.
not sure if that's true or not.
but, i wouldn't have a laptop without one.
they're very good, and they won't hurt your machine at all.
only protect it from hurts.
p.s: i've used both a Speck and InCase hardshell on my laptop.
Both are good, but Speck covers the entire thing while InCase leaves a little bit of lip room.
So I would probably recommend the Speck if you're debating between the two.