All the Ronso and Guado have surnames, although their surnames just inicate the minority tribe they belong to. "Kimahri Ronso" and "Seymour Guado", for example. However, it's notable that Seymour is "Seymour Solus" in the Japanese script.

In Europe, surnames arose when populations grew. In order to collect tax properly, officials needed to know who was who "Bill from village X" just didn't cut it anymore, so people began adopting surnames, based on some aspect of their job or history. Metalworkers often became "Smith", the same was true for fletchers, coopers, thatchers and others. Basically, people took on an extra name so that they could be distinguished from others with the same given name as them.

Spira, however, is a ravaged world - for a thousand years, Sin had been laying waste to human settlements. Resultantly, most communities are small and sparse. Bevelle and Luca are the two biggest cities, and even they are quite modest. Thus, most Spirans probably have little need for a second name. However, in the larger cities, some people might indeed have a surname. This would explain Kinoc. It's notable that Kinoc never gets addressed as Wen, even by his "old friend" Auron. He's always simply Kinoc, which suggests that, even when a Spiran has a surname, it's very rarely used.

Edit: xD @ Sephex