honestly, I don't think the halberdiers are ment to be flexible. They have only one purpose, and thats to hit hard. If you want flexibility, consider the combination of the various units and how they work with each other. The way the Empire army is designed isn't around one solid versatile core unit, more of the cooperation between the different core units (hence the detachment system).
If you're looking for a pretty solid infantry unit, the best one for the Empire has to be the Greatswords. I know they cost twice as much, but they not only have a very killy two handed sword, heavy armor, stubborn, WS 4, but they'll be more then able to hold the line while your knights sweep in on the flank and finish the enemy off. You can also use them as a parent unit, so team them up with (insert favorite core melee unit here) and handgunners and you got yourself a pretty solid and versatile formation.
my two cents
~Valegorn