I think if you are the sort of person whose entire concept of motoring pivots around 'the twisties' then it might be great, but I think for a lot of people - including me - there's more to it than that. I do not want an MX-5/CTR/R32 at all.This exactly. I can't imagine an MX5 being much more fun than the 535d. Sure, it'll go round twisties better, but then what? It is hardly what I would call special enough to be considered the "special weekend toy", when your daily is something like a 6 cylinder 5 series. Unless, of course, it's a purposefully stripped out, track only toy.
I agree with the "toy" having to be special. Along the lines of a 911, or building candy's R34 GTR. These ARE special cars. MX5's, CTR's, R32's and the like just aren't "special" enough to be running another car as a daily drive when they would work fine for this in most cases.
I've looked at 911s - as 996 Turbos are cheap and reasonably safe in terms of value - but by all accounts, while supremely competent, limited fun factor as time goes on, putting them more in to daily territory... but then you'll hit resale value with mega miles.