If you take care of your laptop, when placed on a hard surface, you should not need a cooling fan.
Laptops are designed and tested for a max rated temperature at full CPU and GPU load (generally 48-72 hours)
Laptops are generally not tested for dust buildup as that is a loosing battle since air cooler require air to work.
Many people often forget to clean their laptop heatsinks, or at best look at the outside and think it is clean, not knowing that the dust builds up on the other side.
and thus, those users end up spending tons of money on cooling pads and other products to fix an issue that is free to fix.
http://i.imgur.com/l7txQF7.jpg
Laptop heatsinks clog significantly faster than any heatsink on a desktop computer.
To cool a laptop using a smaller heatsink, a higher fin density is needed, the tradeoff is when you increase fin density, you make it so that the heatsink will clog faster.
Laptop heatsinks will begin losing cooling performance within about 2 months of regular use. and in a dusty home or a home without whole house filtration (eg the kind provided by a central cooling system, results like that image can be had within 5 - 6 months of regular use.