An indirect compensation is that these units require less XP to advance, but I understand your concern too.

> I know you can kinda cheese it by reducing a monster to 1-2 HP and then getting them to attack, but it feels like going against their role.

This is a problem with the XP-per-kill system. Wesnoth could use a variant instead. I use the above strategy all the time to level the healers up.

Note that elven units have slow (their healers), which is very powerful in its own right for getting a kill on a unit. First slow, then deal damage with other units.