Qc2 Bx Qx Ne4 is what I hate playing against - I feel like black gets the initiative, but playing cx feels like "why did I move the queen in the first place!?"
I don't necessarily want the line that's easiest to learn, I want a line that will give me the bishop pair and the center - I think white deserves these two gifts in exchange for black's good position. Maybe the f3 line is more solid than the a3 line?
I don't usually open QP much anymore but if I remember right the trouble facing the Nimzo is that if you want the B-pair you usually take lousy pawns, if you want decent pawns you lose the B-pair and if you try to hold both you usually lose a lot of initiative and face some Kside heat.
I usually went with White Qc2 eventually giving up the QB but keeping good structure so likely draw but with chances if Black errs. I used to goof around with a slow play version I saw, Korchnoi-Petrosian Armenia 1965, that was interesting.
As Black, I recall facing the Qc2 with White developing the QB and getting it to g3 via h4. Though I can't remember the exact details I'm pretty sure that that played better for White. Though I likely misplayed what felt like a very sharp line. If you're looking for a relatively aggressive Nimzo response that might be a consideration.
I probably won't see it the Nimzo with White anymore but this discussion has kind of motivated me to try it more with Black.
As to the question "Why allow the Nimzo at all?" there are a lot of answers -
First, keeping my transpositions straight: If my opponent goes 3...d5, I'm in the main main main line of the QGD, leaving all of my study there accessible (I've worked through everything QGD in Watson, half of Shandorf, and most of the world championship match between Alekhine and Capablanca, so I'm booked to the gills)
Second, I don't like the Bogo - it's just taste....I feel like every try is either ugly or boring. Maybe I'll switch back to a Nf3 c4 d4 setup (many players do this to avoid the Nimzo), but for now I'd like to see if the devil I don't know yields a more interesting game of chess.
Really though, the reason is philosophical - I think the bishop pair is more important than a weak pawn, and playing the white side of the Nimzo is the way to 'prove' this assertion. Of course, it's not really true - structure and the bishop pair are both long term advantages that can both make or break games in their own ways, and engaging this opening head on is a really wonderful way to explore the concepts of structure and material imbalance through book theory AND play.
Why not the Catalan? It's a perfectly lovely opening, and, as a Nf3 enthusiast, I've definitely played a lot of the KIA, but I'm interested in exploring the dreaded Nimzo.....if it's really unbearable, I can easily switch it up and go with the QID/Bogo where I know the structures well.
Thanks for your analysis of the Qb3 line, pfren - Shandorff spills a lot of ink on the idea that the main lines are 'main' because the other lines were found wanting.....this isn't always true, but it's a really important rule to consider when picking an opening (and a key reason that I have no patience with booking up on gambits no one's heard of)