Keep in mind that the meditation bonus is around 30ish mana per tic. Health regen is 2 or 3 a tic.
In other words, if you're not sitting when the tic occurs, you're missing out on an extra 30ish mana you could have had.
30ish mana = Spiritual Ascendance + Steeloak Skin + Clarity Potion
In other words again, if you're not sitting when the tic occurs, it's as if you didn't care that you're missing out on SA + 10 + C potion....
This med bonus is one of the main reasons casters have mounts. The next main reason to have a mount is that you don't get sitting aggro when the mob is in camp. Lastly we use mounts for travelling at higher runspeeds. Some also have mounts, usually horses, for status purposes.
Here is an old link that has mana calculations in it.
http://www.rpgexpert.com/372.html