Clearly this really is a matter of opinion, and I'm fascinated by the number of folk who are prepared to express an opinion on a type of car of which they admit they have little or no experience - makes the results pretty shallow if you think about it.
I have an '88 320i convertible with 177k on the clock and it's a darned nice car, whch DOESN'T leak - no reason that they should if properly maintained - and the fresh air opportunities are just too good to be missed.
I love it, my wife loves it and no-one elses' opinion matters a damn.
We may be eccentric old farts (or so we've been told more than once) as we have two other open cars that are not convertible - that is there is no roof at all.
If you have ever ridden a bike (Beemer or otherwise) you'd realise that a drop of rain doesn't actually do you any harm.
Buy a tin-top by all means if you have to commute in the thing - but if you really want to enjoy the driving experience get out in the fresh air!