I think, it depends in some factors to set the wireless device in acces point mode, or to turn it a router.
agree with all previous mentioned, but another factor, what if within your lan you have a single subnet, and dont have the chance to use aonther one??
the only solution is to extend the lan with the wireless access point.
Of course In the same scenario you can put the access point in routing mode, and perform a nat for the new created wireless subnet, but you are adding complexity to the network, but if you want, all is possible in networking men!.