If by "lower weight" they mean get the car down to 1250 to 1300kg, then by all means go right ahead.
Just regarding the cries about the M brand being diluted (and please stop reading now if you're not in the mood for a bit of a rant), if you want to blame anyone for the current state of affairs, then blame the buying public.
In today's world where a Merc S65, S63, ML63, R63, G63 (or G55? I don't know) and a whole bunch of other AMG cars sell, it would be naive to think that BMW would just keep making an M3 and an M5 forever.
"But BMW could at least use a different naming convention for the non-traditional M cars!" you say? Sure they could've, but the fact is that the M badge carries a lot more weight than "iS" or some other completely new badge.
People want to be able to buy an X5M, or an X6M, and probably an X3M and X1M as well. That's why M is expanding. Not because BMW just decided one morning that they are going to slap an M badge on all of their cars, but because people buy stuff that carries an important badge.
In fact, I wouldn't be at all surprised if they bring out an M5 GT or an M7 in the not too distant future, and I'd be even less surprised to see a whole bunch of these cars frequenting some of the local highways shortly after.
It works for AMG to take virtually the same engine (in different states of tune) and stuff it in almost all of their cars, from the S Class right down to the SLK. That means much lower R&D costs, which means more profit on every unit sold. It also means being able to sell a far wider range of products, which also ups the all-important profit figure. And no, I'm not blaming MB or AMG, they just happen to be a very good example.
To be honest, I think we're somewhat lucky that at least we still sort of get 'bespoke' engines for differing lines of M cars. "M" still means that it's not just a tuned version of a garden-variety car, but that some extra thought went into the engineering of it.
In short, here's why M is expanding:
Typical person A: "Hey bro, I just got me a new BMW, bro! It's a 5 Series Sdrive50i GT. It's going to drop some panties, bro!"
Typical person B: "Oh nice one bro! Is it RWD?"
Typical person A: "I don't know bro, I think it's FWD... What's RWD?"
Typical person B: "That's rad bro! Hey, is it the M version?"
Typical person A: "Wait let me check... No it's not bro, I'm not seeing 'M' anywhere!"
Typical person B: "Aah bro, you need to get the M version! It has an M badge on the back with like 3 colored stripes next to it. ...Bro."
Typical person A: "Right... I knew that bro, this one's just my temporary wheels until my 5 Series M5GT Sdrive9000i gets delivered yo!"
Typical person B: "Bro, that is sweeet!"