Say your cruising and want to make a shallow accent or decent. You can just set it to 500ft/m or 1000ft/m (of what ever) and there you have it.
This is a definition only, your just restating what you wish not why.
Here is a more specific example: Your flying at your mil. power to a fight 50 miles away. A squady tells you that there is a con at 12K, your flying at 10K. You want to be at least as high as that con not to be at a disadvantage but you also want to get there as soon as possible. Therefore you don't want to set your speed to the best climb rate because you will loose a lot of that speed and it will take time for you to accelerate again. You know that a given speed it will take you 4 minutes to fly that distance. So you set your autopilot to climb at 500ft/m and you arrive there at the fastest time and at the right altitude. You do not really want to be keeping your airplane on manual control for thous 4 minutes.
Nope life dosn't work this way. Something will change, want to arrive in 4 mins, no problem set .speed for 50 miles in 4 mins. You will arive on time but at no definition of alt.
But setting constant climb rate 500 would get you to 12 k 4 mins, but you would have no idea what your speed would be, hence you have no idea when you would arrive. And if you did know what your speed would be, you could just type that .speed and be climbing at 500 fps.
If your buddy is in a fight, he most likely would not be at 12k when you got their. By cruise climbing you would also be giving up a lot of E vs doing best climb to 12k then level. I.E. I can not think of a REAL situation where someone would really want to do this.
Also if your coming back to the airport and you know that at this distance, at approximatively this speed (your speed will change but not by much), and this decent rate you will be at the right altitude by the airport so you start your decent at that rate. (I know you know this one because your a pilot as well).
So if you know all this, just type in the .speed and adjust power for decent rate. It is exactly the same as type .climb command and adjusting power for speed.
So I see you asking for it, but I really can see a real use for it. All your examples would not work as you desire.
HiTech