I AM a tank. I am a tank in wow. It doesn't change anything. The only 'game changer' is taht there isn't dual spec, so you need a companion if you actually want to be a healer or tank while levelling (especially since respec costs inflate dramatically). They are plot devices that craft for you, once you hit 50 you will never see them again unless you feel like amusing yourself doing solo quest content or undermanning a flashpoint.
I played both tank and healer and the companion made everything better.
I actually have to tank with a dps companion to keep mobs off them.
I actually have to play as a healer on my tank/dps companions to beat tough fights.
It matters. Have you leveled 1-60 in Vanilla WoW? Completely unviable if you want to get things done efficiently. Companions are a game changer.