A lot of people probably picked Mages because they don't cost any power, so it's faster
What? Mages cost power. You can meet Fiona and Alexius in Redcliffe without spending power, but it costs 15 power to commit to the mages, same as the templars. You just don't get a short scene for free.
And really, once you see that short scene, you're have to either metagame or be (RPing) a complete moron not to side with the mages. Alexius' time magic is
tearing at the fabric of reality. That is the biggest threat we have ever seen in Dragon Age. Corypheus, the Breach, the Blight, red lyrium, all of that pales in comparison to
destroying reality itself. Alexius must be stopped, and that was the only time I really wanted to smack Cullen this game. "Sure, the evil archmage's magic is going to undo reality, but his castle is kinda hard to get into so let's ignore him."
The game really should've made you deal with Alexius right after even if you sided with the templars. He is a threat that should not be ignored, and the entire world being plot armored is the only reason anything still exists after players side with the templars and let Alexius run free. Ironically, a real templar would choose to go after Alexius and stop the mad wizard.