Zippy
Whilst you are correct that the NERC LASs have the MIST display available, it is not the same as having proper weather display radar at the sector.
Firstly, it is only updated every thirty minutes, and glitches sometimes cause an update to be missed, making the "latest" data an hour or so out-of-date. In a fast-developing CB scenario, such data is of limited value.
Secondly, even if we attempt to use the MIST data, it must be printed out for distribution to the tactical controllers, and this process can take anything up to five minutes if the picture is brightly coloured with weather returns (ie. just the time we need it).
So MIST is, perhaps, better than nothing, but no substitute for real-time weather radar that gives a similar picture to that in virtually every aircraft that we are responsible for. Our best and most up-to-date knowledge of adverse weather comes from the pilots up there attempting to dodge it - not a way to plan things effectively.