Bill Casey wrote...
Vega and EDI didn't take spots from ME2 characters...
It's fairly apparent Bioware wanted a biotic, a soldier and an engineer who were guaranteed to be alive...
If anything, blame "anyone can die" aspect of the suicide mission in ME2. That's what made things problematic...
The only real tech power EDI brings is Overload. Practically all her other 'tech' powers could be substituted with a biotic equivalent, and she's a rubbish tech squaddie in comparison to Tali. Kaidan already brings Overload to the table as a Sentinel, and the same power could have been given to Ashely (or an equivalent).
Javik could have easily been the pure Soldier, or the tecchie if the above argument doesn't work for you, while the VS could have filled the Soldier.
Bottom line is that they could get around the issue without creating two unnecessary characters and if they hadn't been stupid enough to force the VS to be missing for half the game.
And if you still don't agree with the above, then how about this: it would have been a great way to show consequences if because of bad decisions in the SM you lacked characters who could bring certain powers to the table. Consequences are sorely missing in ME3. And before you say that would punish new players, the game could have adjusted itself to give the new players the 3 core characters: tech, soldier and biotic.