normally you get it per metre so you will probably have to get 3m which is plenty for any bike! if you can get it in 2.5m then ideal. i havent seen a bike with no cable housing :? the housing will protect your frame and cable from wear (not all wear, just more protection than bare cables). it should also keep your cables lubed i wouldve thought (to an extent) meaning less maintenence
outer will also guide your cables as its more rigid than just the cable
again, i honestly have never seen a bike with bare cables, not fully bare anyway. you do get points on bikes where some cable is exposed, this is normal and depends on the routing. some routing allows for full outer i.e. from bar to mech or whatever, where as some will have gaps in between. if your not that bothered and you have the option, go for full outer, this is the least maintenence method

again, i honestly have never seen a bike with bare cables, not fully bare anyway. you do get points on bikes where some cable is exposed, this is normal and depends on the routing. some routing allows for full outer i.e. from bar to mech or whatever, where as some will have gaps in between. if your not that bothered and you have the option, go for full outer, this is the least maintenence method
