You may think you are just showing a composite image. You may think you're making the game look like what it will look like when it ships. You may think you are helping your product out. You aren't. In fact, you're hurting your product more than anything else possibly could do.
Allow me to give an example from history. Back when "Amped: Freestyle Snowboarding" was announced, a few screenshots were given to marketing to release. The shots, while accurate, were bland. The content was still months away from really being "showable." So, some schmuck in marketing added a lens flare to the screenshots using Photoshop. They released those screenshots in March 2001 during GameStock.
The community picked up on the "enhancement" fairly quickly and were quickly pounding not only the game, but the studio and the Xbox as a result of it. It got to the point where Microsoft had to put the following on the official Xbox web site on March 16, 2001:
Some of the images for Amped released during Gamestock were enhanced to illustrate some features that will be in the final product. While this is a common practice for games so early in development, we apologize for the confusion. No one intended to be deceptive. Everyone was so busy prepping for Gamestock, that we just missed the fact that these were labeled "concept art". Frankly, we're impressed with the skillz of those digital sleuths! Nice work!It wasn't concept art. It was a touched-up screenshot. Now, there were two months left until E3, and it took a couple of man-weeks of time in there that could have been spent doing more productive work to implement a real-time version of the Photoshop lens flare effect so that when people actually played the game for the first time at E3, they'd see that the game looked like the screenshots, and forget the controversy. Morale was shot on the team because they were hearing nothing but bad things about the game because of the controversy. Morale didn't improve until E3, unfortunately.
Now I want to make one thing clear: if your game looks great, but the frame rate isn't quite up there, go ahead and do an in-engine render out for your video while you fix the frame rate. Performance problems can be fixed. Things like what's going on with "Killzone 2" at E3, however, cannot be fixed as easily. When people saw this video, they truly believed that this is what "Killzone 2" was going to look like. If "Killzone 2" doesn't look like this, meaning that the explosions, smoke, carnage, model animations, etc., don't look this good or better, then you're going to be in trouble.