ive seen tons of shamans pre healing tide beams and what not
You're forgetting that all of this is instant cast and that the DK have AMS and boomkins have cloak of shadows or AMS. Stopping them from doing a combo that takes 2 seconds to pull off isn't exactly obvious or easy. There are plenty of maps where close encounters are required. Flagroom fights in WSG/TP. Mine Cart fights. EotS mid fight. You can't exactly spread out to avoid the combo in many situations. Additionally, it's not like you need to capture the whole team. Grabbing even just 2 healers and a couple casters is enough to turn a fight your way. Having to pre-healing tide beams and stop 2 players from doing an instant combo is not what RBGs is about.