If this guy can be dedicated to your schedule it will be faster to have him along.

4+1 tank will always be the fastest setup. In this case if the fifth is another player and he is going to be your tank you have increased flexibility and you never have to worry about icky collection quests to open up a lucrative quest chain. Instancing is going to be faster and if you can spend all your time instancing you will be leveling much faster overall.

This isn't really a parasitic relationship he is seeking. Both players would benefit from it.