In SC pre-made and in organized wb play you target whoever the main assist is targeting.
Period, no exceptions except for tanks to snare/taunt/punt.
If the MA isn't targeting the proper targets in your opinion then you are probably wrong
Target selection is something everyone disagrees on. If you roll with people who properly use heal debuff and other abilities, and you assist the main assist, you will burn through things quickly. If you don't focus, you hurt your team.
In pug warzergblobs, it depends on your role. I often find myself doing my own targeting since the MA will occasionally target the most asinine crap (Volgo ffs is not going down to a handful of pug loldps) and so I hunt for light armored types, lowbies or simply put pressure on healers.
Magus CAN be a threat to pugs who don't have experienced healers and who panic when you drop Pand, Mist, and firestorm when there's also a trio of choppas going wild and a sorc bombing them. The dps isn't much , but it just makes everything that much harder to manage.
In Pug SC? LOL. You deserve everything you get. You might well just burn down the mDPS since no one bothers coordinating. So much AoE flying it doesn't even matter and of course half the team is beating on the front-line tank while he's being cross guarded and healed by two healers.