[goodsound]

One thing to also consider would be the amount of shielding available to which ever cable you choose. Over a longer run you will fight resistance and interference of a greater and greater magnitude as the length increases (also, your wire may run across other power lines etc...). One side benefit to some of the in-wall varieties is a double jacket of insulation. Depending on your preference for speaker wire, this may offer greater protection from interference. Also, given the large variety of cable manufactures available, the costs reflect bulk purchasing keeping the over all costs relatively lower than sometimes buying high end speaker wire and running it through a wall. Also, note the double jacket helps keep wire free from moisture degradation.

Given these points and those of the posters above (code violations) it seems a safer bet to install in-wall rated cables regardless of how easy it may or may not be to actually pull the wire.