The nice thing about Shen support is he does very well with close range, aggressive adc's like graves, jinx and vayne. I do agree both examples do have their benefits on their own end of the spectrum, but that same argument can be had with most any champion. But I digress, counterpoints:
Shen excels with the healing and profit off of Spoils more than Ali because he is not constrained by mana. Alistar can most assuredly beat Shen in healing over the initial trading, but supporting an aggressive lane can deplete his mana pool more than a little fast. You still have the same amount of healing from Spoils sure, but energy is honestly very overpowered by nature.
Leona can definitely cc more but Shen, and using his ult on the carry yields Shen himself kinda useless during the channel. Not going to lie, Leona beats Shen in most everything except the built in healing Shen brings in with vorpal. Yes, it is not much if any at the beginning, but vorpal's heal scales off of Shen max health, so Sightstone onward the healing becomes more and more useful. Laning will definitely suck if you're against a Leona supp, but if Shen didn't have a weakpoint in this role he would be pick/ban for it :)
Completely agree with your thoughts, but Shen isn't as bad off as one would initially think., provided your with an aggressive lane. Hopefully your lane will be pushed far enough that if a cross map ult is needed your carry could either back or wait and play cautiously until your inevitable return. Granted that's in a perfect world, but if your lane is getting pushed enough you can't leave in good concious hopefully that would mean another lane or both are doing better than your current situation :)
