I wouldn't mind being stuck with the either/ or choice. I nearly always leave the 100 pounders behind anyway. About the only thing you can use the 100 pounders on is gun positions, resupply convoys, barges and such.
I imagine the reason for that was the rockets wouldn't fit on the wings alongside 250 lb bombs. I see no mention of the 100 lb bombs on that web site. Wonder if, in the real world, they couldn't fit the 100 lb bombs on with rockets either?