While not irrelevant Owlowiscious' role doesn't really get him anywhere near the significance of Spike. Not only for lack of speaking ability but also for much less frequent appearance. In spite of his unquestionable intelligence Owlowiscious seems to be set a bit more on the same level as Tank, Angel, Gummy, Opalescense and Winona. It may be a bit unfair (since we rarely see him in the role as Twilight's nighttime assistant), but Owlowiscious is considered more of a pet while calling Spike that would sound a bit like snide.