Im finding it hard to comprehend some of the postings on this thread. FlapsOne, what do you think slots are for in the first place, just a bit of a rib-tickler for ATC guys to use to get some laughs? They are there to protect YOU from getting into a situation where you might be less than 100% safe. I'll say that again, 100% safe!
The restriction which is being applied may apply at some sector thousands of miles from your departure aerodrome, so "good on" the neddies who apply that to you as a departure slot time. Maybe they should give it to you as an arrive at position XXX between time ABCD and WXYZ, and let you work the numbers out for yourself, maybe not.
The bottom line is... keeping it ALL - SAFE. No more, no less. Would you fly your kite above Vne in turbulence just to keep YOUR customers happy..I hope not.
Keep a handle on it guys, ATC is not there to restrict, it is there to TRY and keep us all safe.
SuckSqweezBangBlaw