and could you make arguments why are they necessary?
Nobody really says they are necessary. But they aren't unnecessary either. Sure world could live without them ... But since they are available, they obviously are necessary enough to some.
Obviously such cars are "necessary" enough to some since quite a lot of people buy them every year.
Eg. in 2013 combined sales of 3er GT, 5er GT, X6 & 6er GC almost reached 100k unit mark - a mark that was barely exceeded by X5 in 2013. With X4 & 4er GC coming, 100k mark will be easily exceeded in 2014 - with combined sales higher than individual sales of X1, X3 or X5. And eg. 150k sales per year is around 8% of annual BMW brand sales. Not a a bad result for a bunch of niche cars. And don't forget they are profitable too.
So, 100+k customers per year is IMHO a sufficient argument that those cars aren't "unnecessary". At least to some (more 100k per year!).
If find it funny that nobody argues if eg Z4 is "(un)necessary" - since its annual sales are lower than eg the individual annual sales of 3er GT, or 5er GT, or X6, or 6er GC - despite most of these cars do cost more than Z4.
I also find it weird that so called "car enthusiasts" are usually so conservative & skeptic when it comes to new shape variants. Shouldn't car enthusiasts be more open minded & open to novelties ... and not being such purists?
Also ... With so many vehicles available in the markets ... vehicles of various sizes, shapes, prices, features, power & torque outputs etc ... Who is there to judge which ones are "necessary" and which ones aren't? People do have different needs, wishes, preferences etc - and with relatively cheap production niche vehicles have become a trend ... and found quite some customers. Some have even become so popular that they have become mainstream & no more a niche.
Sure everybody has a right to opinion ... And IMHO it's at least a bit ignorant to claim some products are "unnecessary" in general (not to that person in particular - that would be fine with me) despite they sell quite well & find quite some new customers every year. Obviously some people do find them necessary enough to buy them ...
IMHO customers vote with their valets. If enough of them pick such a niche car every year, it's a success & necessity. To the customers & to the company. Customers getting a car even more tailored to their needs & preferences, while the company gets sales revenues & also makes a hefty profit. Win - win.