So wait wait wait what you're telling me is if I assign weapons properly I can have a militia that uses both crossbows and melee weapons?
Sort of. As Maklak points out, they're going to have an extremely strong preference for one of the two, depending on which appears in their inventory first. If you give them a crossbow as the top weapon, they'll shoot it at ranged enemies before trying to bash them with it. They will occasionally strike with a melee weapon if that's given to them.
If you instead give them a melee weapon first and crossbow second, they won't shoot it and will instead start attacking with their melee weapon. Worse, they'll occasionally start bashing with the crossbow in melee.
So the benefit is that if you
do give marksdwarves melee weapons, they will have a small edge over regular marksdwarves because they will sometimes use them in close combat, but only rarely. Truth be told, you're probably best off just giving them crossbows made of a heavy metal so the bashing will be effective. That way you don't have to worry about weighing them down with another weapon, and don't have to worry about them needing two melee weapon skills (hammerdwarf for crossbows I think, plus the other melee weapon's sill).
As an aside, I'd like to point out that there might be something more interesting going on with the number of graspers and weapons / shields being equipped. As Maklak says in his first post, unicorns have a horn that gives them an extra grasper, but Nidokoenig's mod gives ponies left and right mouth graspers, so they can actually carry two things in their mouths. This allows you to give them all weapons and shields, despite only having a mouth. I'm not sure if that has any effect on carrying a crossbow, shield and melee weapon, but probably not since two armed dwarves can use a crossbow and shield just fine.
An arrow to the leg or arm isn't lethal.
Nope, but can be crippling all the same. In my current fort I watched an elite bowman fire two iron bolts at one of my soldiers. The first bolt hit him in the left arm, tearing a ligament and tendon through a steel gauntlet. The second bolt hit him in the right arm, tearing a ligament and tendon through a steel gauntlet. He never held a weapon again in his life, and he was one of my best shots.