Honestly, I'm 50/50 on them.
Positive: I really like when they get characters who could easily fit into the Soul Calibur universe. Characters like Link, Ezio, and Kratos were characters I felt that fit into the universe very nicely. However, I can't say the same for Soul Calibur 4's Star Wars guest characters. I was especially miffed with Starkiller because his moveset could have been great for a brand new Soul Calibur character. Considering that Soul Calibur 4 came out around 2008, I would've preferred a guest like Ryu from Ninja Gaiden, Prince from Prince of Persia, Dante from Devil May Cry, or a Castlevania character (Simon or Alucard) as their game franchises had releases in the same year. I'm sorry to the fans of Star Wars but I just felt they were very unfitting.
Negative: The downside of guest characters is that they can put more time into them and may make the characters overpowered (Starkiller is one of the best examples of this) just for the sake of appealing to the casuals who aren't into Soul Calibur. I also feel that they could take away potential of adding new characters.
All in all, I'm okay with Guest characters so long as they fit the theme of the universe.