Laptops are a little pickier than desktops when it comes to leaving them on. Desktops are built to supply constant streams of air, both in and out. Technically, so are laptops but the heat they generate and the small space they have to work with does leave them with heating problems.
For me personally, I will occasionally leave it on. I have it set to go to sleep after 20 minutes of inactivity, so at the least - the hard drives stop spinning and the general commotion of all the internal parts subside.
When I know I'm not going to use it for the rest of a day or again until several hours later I turn it off.
Laptops are prone to heating issues, depending on your make and model.
So concluding for me, I leave me desktop on 24 hours a day about 5-6 days a week no problem. I added an additional fan unit to help circulation. The laptop is normally turned off - and when it's not in use or I forget, it just goes to sleep.