I know a lot of high-level characters in-game and is always amazed to see them level up in a short amount of time, while I struggle to level at such a snail-ish pace. Their secret? Pilots. No, they're not the ones that fly a plane nor are they a pen brand. They're people employed (in some cases not even paid) by players to hand-level their characters while they deal with real life matters (work, studies, etc.).
Is "Piloting" legal?
In a way it is, as long as you don't get screwed by your pilot. If he or she won't do anything illegal (botting, hacking, exploiting, or scamming other players) while he or she is playing your toon, it's basically okay. I've had a lot of friends who lent pilots their toons and got banned. They would try to convince the game developers/publishers that they were innocent; that rather it was the pilot's fault. Unfortunately, that statement is extremely hard to prove as there's no way for the gaming publisher/developer to know whether the person sitting in front of the monitor is the real player or not.
If you read your game's EULA (End-User's License Agreement), you maybe familiar with the following (or something like it):
Your Password is to be kept confidential at all times and you are solely responsible for the security of your Password. You may not disclose your Password to anyone, or allow your Password to be used by anyone other than yourself. YYYYGAMES.COM LTD is not responsible for any harm that may result to the Account (including without limitation the deletion or modification of characters in the Account) as a result of a lost or shared password.
Although it's stated that you "may" not share the password with another person, it doesn't state anything regarding it to be illegal as "you are solely responsible for the security of your Password." In short, if something happens to your account/toon, it's your fault.
My Opinion on Piloting Characters
I've been playing games since Pac-man and the Atari came out in the early 80's and I always believed that if you can't play the game due to extraneous circumstances (studies, social life, family, work, etc.), then you shouldn't play. If your character levels up because someone else does it for you is almost as good as asking a powerlevelling company to do it for you. Besides, I hear the same reasons from botters/3PP users:
"I'm doing this (botting) because I want to level up faster but I have a life, etc...."
It may not be cheating as per company policy, but it's more like cheating yourself out of a good playing experience.