It is more unprofessional to not file the level you actually want to fly at.
While I do agree that the dispatch system in the US where there is close communication between the dipatcher and the flight crew has its advantages, this is not how airlines currently operate. So, if we refile in todays environment just for a slightly lower flight level for a short period, CFMU will certainly issue an outrageously late slot that no one can deal with, so we don't have the flight plan refiled but rather ask for a level change enroute. If we get it, fine. If we don't, usually a valid reason is stated and we do accept it. In spain, we never hear a valid reason, it is only "stand-by".
The window is there solely for ATC use, to allow for taxiway and holding point congestion which might prevent an aircraft getting airborne at its Calculated Take Off Time (CTOT). ATC can, and often do, apply their discretion of course and use it to make up for operator shortfalls, but that's not ultimately what it is in place for. On that basis, pilots have no right to demand its application for their flight.
Well, operators also need some flexibility. We provide a service for our passengers (real people), You cannot expect us to forecast precisely (to the minute) hours in advance when an aircraft might be ready. ATC is there to help airplanes operate in a safe manner, not the other way round (e.g. airplanes are there as something for controllers to play with).