We have had this discussion with VOSA too (and we have 2 MOT stations!)
As Lucasmax pointed out it is only directly a failure if the car has power steering as the mountings for the cooler pipe are within 30 cms of the corroded section.
If the section is holed but not excessively its a pass and advise but...
If it is fractured or missing sections it can fail on...
The bonnet catch will not effectively latch the bonnet closed. (The centre catch support mounts in the middle of the crossmember)
The headlamp aim/security and possible sharp edges/projections can be used as reasons for rejection in extreme cases too.
The reasons for rejection in the inspection manual have not changed, they have been reworded to make them easier to relate to more modern cars.
MOTs aside if your crossmember is rotten your radiator might be damaged or you might catch the gearbox support on something, not good!
We charge £150(inc. VAT) to supply and fit K11 crossmembers but we do about 1 a week so we're good at it, LOL! £200 is probably about the average.
Or you could become a member of this site and look at my 'How to' guide...
http://www.micra.org.uk/threads/39325-How-to-replace-your-front-crossmember!
:blush: