The section in the ops manaul at bmi (all fleets) reads as follows:
(slightly edited for brevity)
Take off and landings by the co-pilot are always at the captains discretion. Unless the commander is a training captain OR the co-pilot is a senior first officer (in bmi this means you have passed a command assessment course) the commander will handle the aeroplane in the following circumstances:-
1. Operations to and from CAT C or B airfields where weather is a significant factor in planning the approach or departure.
2. X-wind greater than 2/3 of the maximum allowable for the conditions.
3. When the weather is close to CAT1 limits or a non-precision approach is being flown to limits.
4. When the take-off RVR is less than 2x the limit.
5. When there is a primary aicraft system malfunction.
Sounds like quite a long list but in practice the only ones that come up are the x-wind and RVR limits. The others mainly sort themselves out. In addition if an autoland in being flown the Captain is always PF.
I do agree that FO's need to practice in bad conditions but you need to balance this against the fact that in many companies there are often a large number of inexperienced FO's and more importantly, Captains. Imposing some sort of limit can take the pressure off both these groups of people, some new guys are pretty relieved that they are not expected to land the a/c in a howling 35kt x-wind and equally many new Captains will be pleased they don't have to supervise the same.
It's a tricky one really but in my experience the limits we use seem to be sensible and work quite well. If it were my descision I would lift the restrictions after perhaps 3 years and/or 3000 hours rather than waiting until passing a command assessment. We have loads of FO's who have been with us for years who can handle the aircraft at least as well as I can so why not let them get on with it?.