But given that they're continuing to diversify within each series number, even the new ones (eg. Gran Coupe), and unrelated models like the Active Tourer...why did it have to be new numbers? Once again, it's not like it matters for anything, but it's comical.
New numbers for lot's of reasons, there would be 7 different body styles of 3 series now, splitting the sportier variants in to the 4 series range makes the 3 series range less confusing, differentiates the sportier models more (and in turn allows for greater differentiation), and allows them to charge a little more
The only reason they "needed" the 2- and 4-series was because they revived the 6-series, and later decided there should be "logic" to its even number.
Since the introduction of the 3, 5, 7 nomenclature the big coupes (i.e. the 6 series (E24, E63/64, F12/13) & the 8 series (E31)) have carried an even number, this is not a new thing. The reason why the earlier 2 door 3 series versions (E21 and E30) weren't differentiated as much, was because they were 2 door Sedans,
not Coupes - the 3 series was a 2 door before it was a 4 door. Once the idea of a 3 series Coupe became established (E36 generation), it became difficult to change. During the E46 generation there was talk of the next gen (the E92/3) being called 4 series, hence the fact that the Coupe looked nothing like the Saloon. BMW obviously didn't bite the bullet then for whatever reason, but decided to with the latest gen. Ironically they decided that this time around they would make the cars more similar, despite the badge change... I can only assume this was to ease the transition.
The separation of the 1 series coupes again allows for greater differentiation, maybe not as apparent in this gen, but when the 1 series goes FWD next time around it'll probably make more sense.
.. The 2 series Active Tourer however... well, I disagree with the naming there. And based on what I've seen, the decision was made very late. As I said, my belief is they did it to clearly position it against the B Class Merc. But it makes a mockery (even more so when the 7 seat 2er arrives) of what they tried to do with the 2 series badge. If they didn't want to diversify the 1 series badge, they should have gone with the often suggested V badging (i.e. V3 etc.) or similar.
It's still infinitely more logical than giving your cars names...
edit: As I re-read this I appreciate it might be confusing as to how they arrived at most of their decisions, but I still think it's logical.