The difference in Swtor to "other" mmo's is that threat is not instantly generated at 300% here, it is very hard to keep aggro on 4-5+ mobs. Add to the fact that most non-elite mobs can be killed in seconds it is just more practical for the tank to focus on the strongest mobs to build the threat whilst your dps kill the weaker ones.
Okay, so that's interesting. I'm still trying to wrap my head around this.
Would it be correct to say (as regards other MMOs):
1. When fighting a group of mobs there aren't a lot of weak ones.
2. The other mobs take a long time to kill.
3. If the dps tried attacking the other mobs, they would get overwhelmed and would die.
4. It's easy for tanks to hold aggro on the entire group.
5. So killing the one the tank is attacking will ensure that only the tank takes damage during the fight?