Is it safe to always leave a Macbook Pro on?

From a software standpoint, it's perfectly fine to leave your Mac on continuously. It can even be considered recommended, since Mac OS X has a number of optimization scripts that run on a daily, weekly, monthly basis, but can only do so if the computer is on.

From a hardware standpoint, it depends on your model. If you have an older (pre-2008) MacBook with a removable battery, leaving it on and plugged in continuously will kill the battery quick. In that case, you should remove the battery from the machine, only putting it back in occasionally, charging it to around 60%, to keep it in good working order.

The new MacBooks with non-removable batteries are designed to be able to be left plugged in constantly. The battery has circuitry to optimize the charge cycles so that the life of the battery is not adversely degraded over time (more than just normal use causes).

Sure

The current uptime on my MacBook is 21 days.

One thing to note:

• I wouldn't drain the battery like that. Lithium ion batteries don't work like NiCa batteries with memory. If you have it continuously plugged in, charge to 60% and then remove the battery.

Technically, yes, but from a long-term maintenance standpoint, I would reccommend at least rebooting once a week. I reboot all my Macs first thing Monday morning, after the Sunday night system maintenance runs, and I never have problems with crashes, memory and the like (unless, of course I do something stupid while programming). Even Mac OS X needs a quick refresh every now and then.

A laptop is really not that much different from a desktop computer; it's still a hard drive, ram, cpu, keyboard, and mouse just in a compact form factor. Anything you would do with a desktop machine, do with your laptop.