Bows are best for mobile field armies, crossbows for sieges. Bows are fast and accurate, with lots of ammo, really good for fighting on the move and for suppression shooting(really damn high inensity leaves many enemies too busy dying to actually harm you). Crossbows are a siege sniper's weapon - devastating in field volleys, for armies that don't move much, and are much better against shields and in close ranged volleys than bows. Personally I use a crossbow only with a char that focuses more on support skills and for whom I wouldn't want to spend a few points for PD.
Although currently all my chars focus on leadership(STR,AGI and INT at 9), they still have different weapons(depending a lot on the troops I lead, since I prefer to be uniform with them):
the one that leads merc crossbowmen fights with a spear and xbow;
the one that leads Nord veteran archers fights with a bow and 2h axe(although I too dislike the way the long bow looks, especially when on my back);
the one that leads Nord warriors fight with a shield, mace and stones
little clarification on the throwing rocks - most thrown weapons are wildly inaccurate, with really low ammo, are slow and displace the player too much when used(thereby decreasing accuracy even further). Personally I find all the throwing spears in this game are unrealistically heavy, slow and inaccurate(as opposed to the axes). Stones, on the other hand, are fast, accurate, displace very little and come in decent quantities(two large bags means 40 shots for me). While not a killer, they're good at disrupting the attacks of enemies, leaving them vulnerable for my own troops.
Now, I'd certainly like it for rocks to be even lighter and for slings and slingstaffs to be in the game. The main reasons for crossbows and firearms to make bows and slings obsolete were:
a)in the later Middle ages, battles were mostly sieges. And crossbows and firearms were much more convenient for that due to the availability of cover. Although it should be noted that slingstaffs really shined in that age - they were extensively used in sieges to hurl various projectiles over obstacles, including explosives.
b)like mentioned, bows and slings really take a long time and lots of practice to get good at, while crossbows and firearms don't. Give a peasant a gun or xbow, make him practice for a few weeks, bring a big bunch of them to the battlefield and their volleys will take out almost anything.
