A lot is being said about operating at or near to runway capacity, which I'm not going to argue with. The point here is that Glasgow is nowhere near operating at runway capacity so these arguments are not applicable in this case.
Anyone who believes that conditional clearances have not been a contributory or causal factor in runway incursions is either burying their head in the sand, or being deliberately misleading. I can recall 2 such incidents in the recent past which simply would not have happened had a conditional clearance not been issued.
Conditional clearances are not inherently dangerous, but they do introduce an extra risk into the operation which cannot be justified by anyone who puts safety above all else. Does anyone here recall the phrase "SAFE, orderly & expeditious" in that order?
Expedition may suffer slightly if you are at or near capacity, but safe ATC isn't about just firing off as many aircraft as you possibly can, regardless. At least it hasn't been so far in my 20+ year career, and I sincerely hope it never will be.
As for operating GMC & AIR plus vehicles etc...If you're doing the work that would normally be done by 2 or more controllers, then anyone who thinks they can flash traffic around like they do when the positions are split is just an incident waiting to happen.
Ultimately the outcome of the trial isn't going to affect the way I control. I haven't used conditional clearances on the runway for years.