It's worth noting that spears aren't terribly good against anything that doesn't care about its innards- the undead topping that (admittedly short) list. I prefer swords to spears, since you get all the stabby goodness with a sideorder of slashy, and in Adventure mode it's much easier to take advantage of pommel strikes and blade slaps to get your requisite bashy. Swords' biggest problem in Dwarf Mode is that the dorfs aren't quite bright enough to take proper advantage of the sword's versatility.
To briefly boil down the thread, Slashy is good against foes your own size IF they are underarmored- in this case, underarmored means their armor is weaker than your weaponry, and as dwarves are the only ones with access to candy or steel, this category should include any given sentient or wild animal. Stabby is good against things that care about their intestines, and while it likely won't kill as fast it is a more sure way to make somebody bleed out- as noted above, it is not so good against things that don't care about their internal organs or blood loss. Bashy is good against armor and highly skilled enemies, because the accrued incidental damage from glancing hits is greater than that imparted by slashy/stabby. It is not so good when your dwarves are at a significant size disadvantage, or when your enemy doesn't care about the gradual liquification of their outer flesh (again, the undead.)
I would also caution against combined arms WITHIN a squad- in my experience, this drastically reduces training time (especially as regards weapon skill) since each extra weapon means another skill to train, and dwarves will train them evenly. I prefer an assortment of smaller squads- I often only bother training 2-3 different weapons (plus crossbow) so that if one squad gets decimated, I can promote a skilled member from another squad to train the noobs in the new squad.
Huh. This post is longer than intended. SO BE IT