You combine wireless channels to increase effective bandwidth. However, by doing so you expose such a combined channel to interference from other APs using any of the subordinate channels.
If you're in a location were you're the only user of the wireless airspace, and if you need the extra bandwidth of such channels, and all your equipment can take advantage of this, it might make sense to try it.
Personally, though, if a wireless host really "needs" that much bandwidth, I believe that perhaps such a host should use a wired connection. Remember, even with the additional bandwidth of such combined channels, there's all the other issues, of wireless, that impact effect bandwidth.
