There is nothing wrong with playing Runescape either as a hardcore gamer or casual player. I think Runescape actually is so popular for that reason. You don't have to grind to 80, or spend hours of raiding to get gear. You can hop on whenever you want train some skills, then hop right off.
In terms of getting addicted, I think it's all about managing your time and setting your priorities. If your busy and keeping your mind occupied then you shouldn't have a problem with your addiction. I only play when I have no school work, my friends aren't doing anything, and I'm bored at home. When you quit cold turkey you're more than likely to come right back unless you pick up another hobby/activity.