I'm not a bearing expert but I'm a bit confused, I thought most cartridge bearings are sealed?
Perhaps you mean the cup & cone (angular contact) bearings Shimano still uses in their wheels?
According to Shimano this is why they use them:
Angular contact bearings of the cup & cone type offer greater strength than sealed cartridge industrial bearings due their ability to displace lateral and vertical loads more effectively for super smooth rotation and longer durability. Angular contact bearings also allow easier maintenance, adjustability and serviceability.
From what I understand, cup & cone are going to have more friction losses than a radial bearing under normal use (eg vertical and not lateral load) unless they are set up very carefully. I haven't done it myself but it sounds like the Shimano bearings can be set up by feel with a high degree of precision to reduce drag as much as possible, and are easier to dismantle and service.
I have a set of older Dura-Ace wheels but I'm considering going to a sealed cartridge bearing hub for my next wheels because frankly I would rather just replace the cartridge bearings every X years than fiddle around, but I'll rebuild the Dura-Ace first to see how much of a hassle it really is...