Furthermore, why would you want to waste any of Rodriguez's innings at Norfolk?
This is a good question. My answer would be that the question might be partially moot. I think we can manage his innings early in Norfolk in a way that may not actually reduce the number of innings he can pitch once promoted, or at least not by a 1-to-1 ratio.
I think that not all innings and outings really count against whatever target innings number you are working towards. I think there is some merit to the idea that if you're only going 1-3 innings, you can do that as often as you like without much concern for season-long wear. So if he pitches a handful of 3 inning, 50-pitch stints over the first 3+ weeks in Norfolk before a few longer outings to ramp up to being capable of going 6/100, I think that despite pitching 25-30 innings in Norfolk you may only lower the number of innings you think he's got left in his arm this year by 10-15 or so.
You could absolutely do the same thing in MLB, but its a lot easier to yank a SP after 2 or 3 innings for individual reasons at Norfolk than it is at MLB where you (should) actually care about the outcome of the game.
Also possible this theory is garbage, and that all innings count the same. But I think injury avoidance is more about limiting fatigued innings than simply counting all innings.