Roads are a lot more crowded today than they were 50 years ago, so passive safety (strong frames and airbags) is a whole lot more important than is used to be. Even for sports cars. All the agility in the world won't save you when some bloke yapping on a cell phone blindsides you with his 5-Series.
I hear what you're saying but what if it's a 7 series, or a Hummer, where do you draw the line?
I have a friend who's Elise was totalled by an SUV, luckily it hit the passenger side and there was no pasenger. If it had hit the other side he would probably have been a gonner, but does he regret driving a lightweight car just in case he might have a crash, no.
It's horses for courses in this debate, too subjective for everyone to agree, personally I just think the 1 series could be a bit lighter. The 135i should be for the masses, but they should do a stripped out version for the enthusiasts, like Renault do, and actually charge less for it 'cos it's got less in it! (like renault do)