From a review on the 6 series, BMW's comments about folding hartops:
"At a time when other manufacturers like Peugeot and arch rival Mercedes-Benz are moving to hard-top convertibles, BMW chose to go with a soft top. BMW claims a folding hardtop reduces boot space and adds too much weight, thus "detracting from the car's dynamic driving pleasure", yet the 6 Series convertible weighs 200kg more than the 1620kg coupe. This extra weight is largely due to the extra chassis rails and bracing needed to compensate for the missing roof -- which is a fundamental structural member. "