Start a conversation about digital manipulation and you are likely to get any number of people 'hot under the collar'. Personally I quite often use digital manipulation. I lean forward and remove a blade of grass just in front of the lens, or reposition an errant twig by hand (digital) and sometimes with my industrial photographs I completely arrange the objects in the image as one would do for a stil life. Gardeners use digital manipulation when they weed and prune. A car afficionado who take armour-all to his tires isn't returning the car to new condition - it didn't look like that new either. A woman applying makup is digitally manipulating, so lets not get silly over this whole process.
All that's happened is that post image recording type manipulation has been made a whole lot easier so many more people are doing it.
If someone takes a sunset picture and shows it to you - he could be saying, see, nice picture, or he could be saying, see, I got luckier than you, or see what you missed or I want you to acknowledge that I had to hike miles, wait hours and suffer to get this image, aren't you impressed.
Frankly, in each case the photograph serves different purposes. If the intent is to impress about the remarkable conditions, then any manipulation could certainly be seen as cheating, though had the photographer loaded the camera with Velvia, then arguably he was cheating from the start. The only scenario I have the remotest interest in is the one in which the photographer wants to show you a nice photograph - he isn't inviting you to go visit the spot with him, he isn't bragging about his efforts and the image isn't functioning as a reminder of a good time had.
If someone shows me a sunset image and the colours look unnatural - this could have been due to freak weather conditions, but if they look "artificial" to me, it doesn't matter that the photographer swears this was what he saw. On the other hand if the colours look natural but were in fact manipulated, does it really matter?
Frankly, anyone who makes the claim to not do any kind of manipulation should be called a Recordist, not an artist. At best you might think of them as a craftsman. Does the world really need "I was here and this is what I saw" kind of photography?
Art is about interpretation. There has to be something of the self in the image for it to mean anything.