Full disclosure: I have done it too.
Is this caused by a lack of creativity? Perhaps its just a natural path that Christian comic artists will go through.
In life, I find the most overt Christians are typically hippocrates. For example the speeder that ran me off the road, with the bumper sticker "Jesus is my co-pilot". Another example: The loud Christian co-worker with the jean jacket with more Christian patches then you have time to read at work, but he is also known as a liar. Calling out sick, but posting vacation pictures on his FB page demonstrating clearly where he was.
My point is that this overt passive symbolism seems like a shield for liars and hypocrites, and people know it. So as people that trade on iconography (see: Understanding Comic by Scott McCloud) do we raise similar hackles on the back of reader necks when we do this?
On the other hand, the most sincere Christians I have met don't have stickers and patches. You know they are Christians by their behavior. Can we play more subtle, or are we stuck with the overt? Don't get me wrong the Gospel can only be conveyed overtly, but a bumper sticker is not the Gospel. Icons are not the Gospel. Can we be overt when we have them firmly in the grasp of the story?
I know this will open me to scolding, but I hope that perhaps this might be a good conversation to have.