Yeah, and if only Legion is alive the Quarians die off unless you betray him. Garrus I feel is just made to feel like he isn't there, no one takes his place, you just continue your mission on Palavan's moon without him.
First, if they died, Garrus won't appear in Palaven and won't be an squad member. Tali will be replaced by admiral Xen in the mission on the geth dreadnought and the peace between qnarian and the geth will be more impossible (Legion must be alive too if you want the peace). See the article Priority:Rannoch for more details.