Hi alosaurus
Low Vis ops can be very frustrating for all concerned.
As I'm sure you're very aware, there are at MAN differing categories of operation: one where all exits are available and one where only BD and A are available (for 24R). These differences were introduced by ATC to increase capacity when the cloud ceiling is the sole cause for the low vis ops. The visibility was determined to be good enough to use all of the exits. When the cause is low vis., however, things slow right down.
If we're trying to get a departure out between every arrival, spacing is based on prevailing conditions and experience. It usually works out at about 10-12 miles between arrivals in CAT2. This is based on aircraft exiting at BD. When the inevitable happens and traffic misses it, the controller is left with traffic lined up and a probable missed approach. Do we hedge our bets and not line one up only to be left with a missed hole when you make BD? No we go for every reasonable one and sometimes it works and sometimes it doesn't. Hence the frustration.
If we were to plan for the worst case scenario every time, arrival delays would be huge and extremely wasteful with arrival spacing of 15 miles or more required but not always needed. There's nothing worse for you than tours of the TMA and possible diversions, and there's nothing worse for us than one mile less space than we end up needing.
What is needed is the ability to use all of the exits in all conditions, but that comes down to £££ and whether the airport operator think that the not insubstantial investment is worth it for the inconvenience and frayed tempers encountered on just a few days a year. I think the answer is a resounding "no".
Hope that helps fill in the blanks from our side of the mic.