Notebook coolers don't really have that much of an effect on some notebooks, unless you have a really anemic fan and you need your laptop raised a bit, I really don't think it's worth buying one just for the possible cooling benefits.
The main problem with cooling a laptop is the fact that there isn't enough space to put anything better than 1-2 heatpipes and a couple of aluminium fins in there, extra airflow won't fix that.
I bought a Zalman cooler and the temperature difference was almost 0, due to the fact that my laptop's cooling ability wasn't limited by the airflow, but by the few heatpipes and aluminium fins it had, if I would buy a notebook cooler, I would buy it mostly because it's a nice stand, offers some extra USB ports and if it has a fan, it should be silent. Be weary of notebook coolers with very big fans, my experience with >140mm fans is that a lot of them come with crappy bearings and buzz at low speeds.
Most intake vents on a laptop are located on the bottom of the laptop. while you can use things like books, dvd or CD cases to prop the back of your laptop up so the vents arent kissing the desk, there will still be a small pocket of dead air underneath it and that wont help the temperatures of your laptop.... Having a laptop cooler with small fans to keep the air moving can lower temps quite a bit even though the results might not be as drastic as one would hope and like to see. theres no denying that it prevents the laptop from overheating so long as there are vents at the bottom.
My last laptop always used to go into thermal shutdown when gaming untill i got a copper shim for the GPU and a laptop cooler. so for me, a laptop cooler was quite effective.
Im using the Zalman NC-1000, bought it about 3 or 4years back and its still worth every £ i spent on it though I would have prefered to pay a little less.
If you ask me all these coolers are a waste of money. If you want cooler laptop, buy SSD instead. And clean it regularly so it's not all filled with dust.
your thinking is flawed.
A mechanical HDD doesnt generate anywhere near as much heat as a CPU or GPU. However it does contribute to the heat and while switching from a mechanical takes that out of the equation, you still have the rest of the setup to deal with. (NB/SB chipset, GPU/CPU or other compenents that might generate heat)
having a laptop cooler keeps the air in the deadzone underneath the laptop flowing and flowing air means cooler temps as the cool air is sucked into the vents and not the warm air.
There are some laptop designs that dont work well with coolers as the intake vents arent located at the bottom of the laptop but at the side.
however those seem to be very few and far between these days.
If laptop coolers made no difference at all, nobody would make or sell them.