It makes the most sense, though, and it has since ME2 expanded the party NPC list. It's pretty much what they did from BG1 -> BG2. The majority of CNPCs from BG1 either made cameos or were killed off. Only the important ones (eg family, love interests and comedy relief via Minsc and Edwin) made it to the sequel as party characters. And considering it's highly likely that companions will have their own subquests (with or without "loyalty" mechanics) the permanent ME3 party pool is probably going to be ME1/ME2 standard.
If I were to gamble I'd say there are going to be sections where you can run with a former party NPC, but only for that mission (ala Lair of the Shadow Broker). A lot of people are going to be unhappy with this, but I don't see Bioware really shaking up their game design to avoid it with the given amount of development time they have, and the proven success of their formula.