doubleohfive
Fleet Admiral
I think it's just after-the-fact. Why would they put Spock's face into DS9's credits anyway? He was only mentioned offhand in one episode, and never appeared in it. There's no reason to have an easter egg like that. Especially when it's almost impossible to see.
1) Because it's fun?
2) As has been pointed out, Spock appeared on DS9 via reused footage in "Trials & Tribble-ations."