@ DP: True. In a scenario where Garrus and Tali died and the player never bothered to get Javik's DLC pack, they'd be down to Liara, EDI, Ashley/Kaidan and Vega.
@HED: A batarian squadmate would've been a lot more interesting, I agree, but it would've also overcomplicated things. If they wanted to introduce a batarian into the mix, the best time would've been in ME2. There wasn't really time to get chummy with the squad like in previous games and since Vega was Alliance, all Shepard needed from him was to do his job.
Also, I didn't like Vega initially, as I said in my blog posts... too dudebro... but he's not a bad character, he was the right kind of character to have when you need an extra person on your squad that doesn't end up taking the spotlight off of the characters from past games who have a history with Shepard.