Brade fade - an accurate username indeed!
I wouldn't say we are there for your convenience, but we are there to provide you with a service, indeed the very best service we can - and from most people's comments on this and other threads, we generally manage to do this.
We would be more than happy to do away with slots and have total freedom, but that ain't possible. So we need a bit of flexibility, that is the -5/+10. What happens when an airline uses this small window for their own commercial purposes - increased pressure for us to get you to the hold and airborne in a limited amount of time and for you to achieve this. It all adds up to reduced safety.
Sorry if that inconveniences you!