pull offs only give a legato sound if you want them to, and try hard enough to achieve it. Me, i do pulloff triplets, and it sounds semi-legato, semi alternate picked, which is a really cool feeling.
but first, you have to learn to pull of single notes before you can do doubles or triplets.
in my opinion, alot of it has to do with your dexterity and how strong your fingers are, if you have nice callouses, etc, if you want a really groovy pulloff. you will notice they get better the more you play.