Squad mates

Besides not having either as a squadmate, you mean? Tali is necessary to resolving the Geth-Quarian conflict peacefully; without her (and Legion) they will fight and decimate each other.

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.

ME:1 Kaiden Or Garrus For Romance Liara

ME:2 Zaeed Mostly Or Legion For Romance Miranda But Keep It From Liara From Know I'n ME:2 Maybe 3

Never Played ME:3 But My Favorite Squadmate Would Be Javik

Tali will not be able to set peace with the Geth and the quarians and if garrus does which is sad I don't know for sure but I think you get the primarc? Correct me if I'm wrong