Hi all,
Crossbows are hypothetical in Middle Earth but I agree Dwarves (or possibly bad-guys) seem likeliest candidates if anyone used them.
They do have some advantages over longbows. The key point was that crossbows could be effective when used by poorly-trained troops but longbows needed archers with practically lifelong experience to be most effective (see Elves
). The crossbow can be more powerful, but is slower to load, however it can be kept 'cocked' for a while. This could be very useful in sieges or perhaps underground fighting when waiting for an appearing target. The relatively flat trajectory of the crossbow bolt means it can be easily fired indoors, and if necessary through small holes etc.
Saying that, the Iron Hills Dwarves included some bow-armed troops.
As Elmo commented, Bard disparaged the Dwarves' military abilities above-ground but I think he's referring to their tactics, being unaware that the Allies had concealed the bulk of their forces in ambush.