Back then to me it was quite clear that "advancing the main story" and "maxing out source points" don't *have* to go together, even though they are intertwined. You can tell the latter is pretty major though, considering how you are constantly reminded of it as you progress through Act 2 - it's hard to miss it unintentionally.
Without all 3 points you can't use tier 3 source abilities, but that's about it. It's not like a game-breaking oversight or anything. To be honest, I played on Tactician and I didn't bother with those skills much, because they were so overkill. In most cases, anyway. It's true some fights I was glad I had all 3 points, but generally you can totally make do with 2 points. Having only 2 points might actually give you a more solid tactical challenge once you get decent at this game.