Good point. But at least Spock's *half* Vulcan. Uhura is NONE Vulcan. If they were to have kids, that would not help the Vulcan species any.
And I think the Vulcan species would benefit from Spock having kids with a Vulcan woman (his human side would be diluted away in a few generations). He is the son of Sarek, after all. Sarek surely must have some clout with what remains of the Vulcan government (who knows, Sarek could *be* that government!). And Sarek would most likely advise Spock that it's logical to have more Vulcan kids.