For many people who play cRPGs, myself included, precisely for roleplaying reasons, using mercenary companions is flat-out not an acceptable option. We want a custom PC, but non-custom companions.
How well does that idea work with "I killed Wynne, WTF! I have no healers anymore, BioWare you're shit at designing games!" that hiring/creating mercenaries help fix?
Moving from Act 1 isn't the time you might lose a companion, the other cases will all be role-playing reasons/choices.