Actually, "cleared to" does not mean direct. It is a change in your clearance limit following that same routing that you were already on. You need the word "direct" in there for it to change the routing.T
The FAA puts readback mistakes 100% on the controllers AND 100% on the pilots. ATC's failure to catch a pilot's incorrect readback does not get us off the hook for not following the actual clearance given.
We can turn the sign off early in the climb if we think the ride will be smooth. The problem is, there's rarely anyway to know if the climb will be smooth.
As you climb, you are climbing through layers of atmosphere with significantly different winds. When these wind changes occur gradually the ride will be smooth. If the wind changes occur over a relatively small change in altitude the boundaries between differing wind speeds and directions will be turbulent and we have no way to detect them. Once level at a cruising altitude we can get a lot better idea of what kind of ride to expect so have more confidence that we won't be surprised by turbulence with the sign off.
It's about spacing, not a controller's clearing multiple airplanes onto the runway at the same time. There are very defined lines drawn as to runway separation and we typically operate with as little as ten to twenty seconds of "padding". That doesn't mean that we're that close to an unsafe situation, we are that close to hitting the defined lines that maintains safe separation. When it becomes apparent that the line will be crossed, the arriving aircraft is usually sent around although it can sometimes be resolved by withholding a takeoff clearance.
Flying the 737, I probably have two or three ATC generated go-arounds a year.
Yesterday at LGA, where they land and depart on intersecting runways, I heard an AAL flight have it's takeoff clearance cancelled twice between, in each case, it did not start rolling fast enough to get through the runway intersection with the required spacing on the arriving aircraft. I has pleased that the LGA controller was able to cancel the takeoff clearance in each case as we would have been the aircraft to have to go-around on the second instance if she had not.
If you're interested in the details of the runway separation requirements they can be found in
FAA Order 7110.65W which is the regulations under which ATC operates.