Alignment is key because it dictates personnel for the defensive front you have out there on a given play. When people talk about whether defensive linemen are best suited for a 3-4 or a 4-3 what they are really talking about is what alignments they would be best at. A defensive end lined up at 4i is going to have a similar job regardless of what you label the overall front.
It's a passing league these days, and so if you listen to a good analyst break down a defensive formation these days they go back to front. It starts with the coverages (single high, two high, three high, man or zone) and then moves to the alignments along the defensive front. That's how opponents will be breaking down our defense and they won't care one bit whether you consider our defense a 3-4 or 4-3 or 3-3 or 4-2 on a given snap.