A more simple answer is that we are a little lazy, actually that isn't true, we are a little busy !! when you are given descent FL200 Level PEDIG and then the TC Midlands controller transfers you to TC N. What North should really say to you is "Descend FL150 level CLIPY, cross PEDIG FL200 or below". When you have sixty + ac to talk to in an hour that involves a lot of extra words which although technically so, are not practically necessary.
The standing agreements between sectors include a bit which says the receiving sector will not issue any instructions which will affect the expected climb/descent profile set up by the previous sector. By removing the previous level restriction this is no longer guaranteed. In reality it often make little difference but there are cases where the level restriction is required to separate traffic from other routes (ie L9 inbounds to TMA from Bristol sector FL140 40 before OCK, if this is not met it can get very embarassing against the gatwick inbounds southbound at FL150 which transit OCK airspace but do not work the sector !!). In these cases I would expect that the level restrictions are more strictly applied.
Hope that answers the question !!!!!!