If a front goes through, there could be large variation in the QNH over a very short time.
I thought about this too, but why is a TAF QNH that is forecast in 3 hour intervals still okay to use? Surely the METAR QNH that was worked out 30mins ago will be closer than the forecast TAF QNH if you're arriving somewhere, say 5 hrs after the TAF began, but 20 mins after a METAR is generated.