Spears are effective against targets significantly larger than you are. Why wouldn't dwarves use them? There are all sorts of nasties that live underground.
Some thoughts:
* Spears are almost entirely thrusting weapons; this is good for deep puncture wounds against things with vital internal organs, and slipping between the joints of heavy scales or plate armor. The odds of a "kill shot" against various nasty monsters are comparatively good, although still low; a high risk, high reward strategy.
* Spears tend to assume plenty of wood. Historically, most cultures had far more wood than metal, and a spear is an excellent way to produce a hybrid-material weapon that economizes on the metal. This may be somewhat less likely for DF, especially deep dwarves and/or less pleasant embarks.
* Spears should be a poor choice against "homogenous" (or effectively so) monsters; colossi, elementals, zombies, etc.
* Spears and other polearms give an advantage when the foe is at range, and can be kept at bay; but once a foe gets well within their length, they are at a significant disadvantage. Therefore, they're not such a good choice against fast, small, and/or sinuous monsters, highly skilled martial artists, etc.
* A pure spear lacks options, and isn't that great of a tool in noncombat situations. Something like a glaive-guisarme or voulge-guisarme has far more utility; you can thrust, cut, or particularly pull.
Personally, my idealized realistic dwarven military would have three main weapon specialties: a front rank equipped with dwarf-scale bardiche; a second rank equipped with glaive-guisarme with a screw-joint in the shaft to allow use at full extension in larger areas and extending past the first rank, or with the bottom few feet unscrewed to give a more wieldy weapon for closer quarters; and one to three ranks of crossbows for alternate load-fire cycling.
This gives you most of the advantages of the later pike-and-shot squares, but better capable of handing enclosed spaces and with additional protection against close-in foes to prevent fast-moving monsters in tight spaces or the equivalent of the zweihander doppelsoldner in open ground from wreaking havoc amongst your pikes. It suffers somewhat by having slightly less maximum reach and somewhat lower percentage of reach weaponry against heavy cavalry charges, but that's less likely to be a problem for most dwarven polities than a typical Euro-human army.
Note that the "giserne" depicted in the Sir Gawain and the Green Knight (
see key fragment here) is intriguingly more like a longer-hafted bardiche (see the Oakeshotte interpretation on guisarme), but there are hints if you look closely that the reverse side had a warhammer-like striking head. This would be particularly useful in causing shock damage against hard-armored foes that might damage the primary cutting blade more than they took damage from it.