White Knight
Does the climb rate really matter anyway once away from the airfield?
For sure. Remember, we controllers are thinking about ground speed in longitudinal seperation. We mess with your airspeed to achieve distance ie. a ground speed. Eg if the Kenyan 767 takes off right behind you, and let's say you are both speed pegged at 300kts IAS, but after 10 minutes of flying time the 767 is 10,000' higher, there will be significant closing (over the ground). This means you can't have unrestricted climb (a step climb or other workload-increasing activity has to happen). Whereas if the 767 got away ahead of you, he is always going to run away from you- no further action required and unrestricted climb for both (unless you are going to cruise at the same level for long periods).
Also, a lot of protections are built into routes
if you are cruising (hemisherical levels etc). By significantly extending the amount of time you are climbing, you are adding risk.