Looking at it from a safety perspective, what advantage is there in delaying a change of pressure reference when your cleared level/altitude/height requires this new reference to be correct? Anything which is not acted on immediately is liable to omission. Not having the right barometric datum can cause conflict with other aircraft and the ground!
I see postings like: 'We'll pick it up in the checklist passing TA/TL'. What triggers this checklist, especially in a non-EFIS cockpit? If it relies on someone noticing passing TA/TL on their altimeter (no bugs, lights, sounds, etc.) then it is liable to omission, especially if the workload is high or there are distractions. Also, as pointed out by others, effecting a change at TL/TA may be too late when in low QNH conditions.
In the moderately rare case of 'stop climb/descent at FLXX/XXXXft', you can change back to whatever reference you need, there and then.
As far as procedures and ICAO goes, airlines' ops. manuals are generally signed off by their respective NAAs as being compliant with regulation...