Yes, but to provide military support, and in some cases political support one would require the funds from their land. No good being the best military leader if you can't also support a decent levy of knights and men at arms to actually lead. Thus to ensure that they were getting the best military support possible you had to consider the ability of the Lord to ensure the lands granted to them continued to produce the wealth required. This is exactly what Solari is advocating, military support is maximized when the regions, especially towns lands and cities are well looked after by their Lord.
But seriously, do you think that, say, when the Franks sat around in recently conquered lands, they looked around and went, "Now, Joe over there is the best fighter, but not too great at counting sheep, so we'll give him a small estate, and just tell Bob, who is a good farmer and thus going to get a big plot of land, to give Joe some of his excess money." Really? I find that implausible.
Obviously, yeah, a catastrophically horrible land-manager might not be preferred (though I can think of plenty of cases where it wouldn't be crippling: I'm thinking of the Mayor of the Palace phenomenon in the late-Merovingian period particularly), but I think intentionally shifting your dukes out of combat roles...
Well, I dunno, there might be some kind of precedent, I feel like, somewhere in 1300's Italian states, maybe some German ones... but broadly it just seems very strange to me.