Trouble is, there are lots of units out there that won't look to avoid a Halberdier Horde - they can take it head-on and win, even with support. You really expose yourself to this if you bring 3 of them, because you likely won't have anything that can intercept the foe. You want to bring units with different matchups, so that you can handle a wider variety of situations. Consider that a 5-man Demigryph Knight unit clocks in at a similar price to the Halberdiers, and they will crush low-Strength attack spammers, which would deal serious damage to a block of Halberds.