If you want big units of around 30, the long flanks make them difficult to manouver, and easier to be flanked.
What is more, you have to remember that Empire troops are quite average, and against a lot of armies (Elves, Greenskins, humans, rats, undead and sometimes dwarfs) if I put spearmen 7 wide then the 15 attacks will quite possibly add a few points to CR. If I put a warrior priest with GW and IoM in this unit then against undead it is a real monster for the deadites.