Have a look at
this CeHu experiment of mine - I abandoned him eventually because it was so unsatisfactory deliberately shackling myself from training melee skills. If you balance out your EXP budget carefully, I think it's possible to have Bows to 20, Crossbows to 14 and a melee skill to about 14 for Vaults-depth.
In other words, you don't want to 'switch' from bows to crossbows but picking up crossbows as a backup shooter is viable - just don't neglect to also have a bit of melee as further backup. Somewhat redundant and sub-optimal (you could have gotten Haste instead) but if you really want to be shooting a lot, dual shooter skills is viable.