I can't say I've ever tried it but I'd think by clamping two jaws on the same side(only two sides clamped), using a center, light cuts, & very low RPM that you could get retangle stock cut. If it isn't bigger then the chuck you can shim it to be square. Hex can be done. Who doesn't get a 3 jaw handy tho?4-jaw scroll chucks (self centering) are still produced). The only advantage I see with a 4-jaw scroll chuck is if you work with square stock often with the ability to still hold round stock. As noted it can't hold hex stock. It will also hold thin walled work with less deformation than a 3-jaw but not as great as a 6-jaw. It can't hold rectangle though.
I don't work with square stock often so I opted for a 4-jaw independent for the accuracy. I use a 6-jaw the most, with a 4-jaw independent second, & 3-jaw coming in third.
There are also 4-jaw combination chucks that function both as a scroll chuck & independent. Awesome chucks but not useful enough for to justify the high cost.
The only reason it wouldn't hold round stock as well is if it isn't actually round. Rolled stock has a odd shape that wouldn't contact all the jaws. I usually turn my stock then flip it around in my chuck before starting my work with a scroll chuck. I don't do this when using an independent jaw.