DeltaTango,
In the case of Israel, a transition level is specified for descent, so yes, you should go to QNH when approaching the transition layer.
However, many SID's out of Tel Aviv have assigned transition altitudes. This would mean to me that if you're at 3000 feet, and the controller clears you to climb to FL260, then you would immediately reset your altimeter to QNE. Otherwise, the controller would have to tell you where the transition altitude is, and I've never heard this before.
What you are saying is right, you may be recleared to a flight level after being cleared to a lower altitude, and you would have to reset your altimeter to QNE again. However, IMO, that's no big deal, since you're cued to do it. As others have pointed out, there is often no cue as to when to reset your altimeter at a specified transition layer, especially in areas there the transition layer varies from airport to airport; chances are that you'll forget to do so. What's more, the transition layer may be below the 10000 ft/In range checklist. You may not discover that you forgot to reset to QNH until quite low, depending on your company SOP's. There is risk here of CFIT, or at least an altitude bust.
I'm pretty certain now that in the UK, altimeters are reset immediately to QNH upon being cleared to descend from a flight level to an altitude, and it would appear that the UK AIP specifies this. What about the rest of Europe?
Scud