You are quite right that buildings insurance is covered within the service charge, however, it is difficult to ascertain if this covers everything. My understanding is that it may only cover the re-building of the block and fitting of common parts, but not necessarily the fitting out of individual apartments i.e bathrooms, kitchens, etc.
I also doubt that this insurance would cover individual claims for damage to sanitary ware, securiy shutters etc so for the relatively small extra cost I have everything covered. Having said that, it is for inidividuals to decide what level of risk they want to accept and hence what type of cover they want to pay for. Good Luck.
Regards,
Melvin