I did it a lot when I was in the begining (and still do with characters I draw less), and I think the issue is that I hadn't drawn them enough before starting the comic, so they wouldn't be uniform through the pages. Now I have a better grip of them so I have a more consistent artstyle.
I do think the lack of consistency shows that we are amateurs and make our work less professional, unfortunatelly. At the same time, I agree that improvement is different from inconsistency though. Your artstyle can always evolve and change, you can always see that even in mangas (I only can think of Gravitation right now) and I find it fascinating, I love to see that improvement, but changing from panel to panel is... complicated, you know? It kind of shows you don't know how to draw your own character in a way, cuz you're always changing it. But that's my own take on it, and obviously I do that from time to time yet, I'm far from professional xD
And if that affects the comic I can't say for sure, as a creator, I notice those things when I'm reading a comic, but I may not drop the comic just because of that if the story is good, you know? An avarege readder may not even notice it em keep reading it.