Computers can get damage from being hot for extended periods of time. Especially things like MacBook Airs, which don't have much of a cooling system. solder joints can become damaged, capacitors can fail, and many other things could happen.
Whilst this is true, I have tested some case fans that have been running at full speed from a MOLEX connection, 24/7 for 7 years, and they still run fine. They were just like $8 new and both are still running like new.
So the chances of it happening aren't very good.
You're using large case fans, small laptop fans can be damaged much easier.