5mm SMG are pretty light and in combination with lighter ammo which you can then carry in larger amount would make nice cheap alternative to assault rifles.
Bolts weight 0,05 (or 0,1 for special bolts) also crossbows are not really lightweight and I don't recall them heaving burst fire capability what would be the main reason for 5mm SMG, AP cost is also higher, so they are even more limiting then 5mm for weaker enemies, crossbows look to me more like alternative path to sniper rifles especially in combination with special bolts which can have some really nasty effects which is why my 'metal armor sniper' first takes out enemies with crossbows since they seem to do much more damage then all other types of enemies (beside other metal armor snipers).