Aren't they just used only for cheap siege armies though?
Indeed! If you have enough. They’re All Ages so no limit on which map they can be used on vs the Spear technique in All Ages (near unlimited sieges with 30 second recruit time).
“Early Age” use is true if one is only looking at their attack/defense ratings where, depending on your boosts, they are a useful regular heavy unit through maybe HMA, then become overpowered. As a regular unit!
“Higher Ages” Color Guard bonus comes in handy at times though, where that extra 1 health can/will stretch your other units through a tough battle, especially two waves. It’s enough to turn most units from a 2hit kill into a three, or a 3 to a 4, giving you one extra round of attack to finish off an enemy.
Drummer is pretty dumb in any age above EMA/HMA as it’s attack boost is usually outweighed by normal means and/or tavern boost. It just doesn’t make a meaningful enough difference and god-forbid using it with a color guard. The loss of two regular units for the minimal boosts these provide will harm you more than help.