Christian celebrities can serve a good purpose. I applaud their witness to the faith and am happy that their talent as an entertainer or athlete has given them a venue to share this. In today's world, this may be one of the only ways in which non-Christians are evangelized. This fact is sad, but often true.
On the other hand, these people can do damage to the Christian "brand," most especially with outsiders. They may be living in a way that is incongruous with their professed faith, as we have clearly seen with many politicians. Or they may fall off the wagon at some later date, something that happens all the time and in no way negates the truth of our message, but can provide much scandal when a supposed spokesperson does it.
I would also argue that the hoopla over Christian celebrities, which is not necessarily their own fault, can amount to worldly engagement in the celebrity culture. We're so excited to have a role model who has achieved success on worldly terms. It's hard to separate whether we like them for the Christianity or their celebrity talents. Can these people really make Christianity any better?
Lastly, as a Catholic, I find that these people distract us from better Christian role models that we've had all along: the saints. This would include people like Thomas Aquinas, Joan of Arc and Mary Faustina Kowalska, among others. The saints are famous specifically for their holiness, not a celebrity talent. They are also a much more diverse group and arguably more relatable than modern celebrities. I find it ironic that some of the same people putting these Christian celebrities on a pedestal claim that devotions to saints are idolatry. It seems to me that many, but certainly not all, Protestants lack a sense of history. There are biblical times and contemporary times and they don't care how God was at work in-between. This desire for more role models beyond the obvious example of Christ himself leads some people of this type to contemporary Christian celebrities. The closest thing the Catholic Church has to what I would consider a Christian celebrity is Bishop Fulton Sheen. Nevertheless, we don't give him extra special pride of place just for having been on television.
Christian celebrities can perform a meaningful evangelistic purpose in our modern world, but the more overt they are, the more responsibility they have. Fans, though, should be cautious of how they relate to these figures and remember that these people are primarily famous for their celebrity talents, not their Christian witness and that we may have better Christian role models to turn our eyes to.