You might get away with normal letterbox scale with point sampling, but I don't know how all graphics cards would handle that. Depending on what you do you might only need point sampling to solve seams (since it avoids the issue with mid-opacity edges not adding up to 100% opacity) - the integer scale and pixel rounding are just to make sure under all circumstances seams don't appear, and those are pretty good settings for pixel games anyway.
Yes, games can appear less smooth this way, but it's a tradeoff you have to make. The difference should not be enormous. Remember lots of players aren't that discerning, some people happily play bejewelled at 4 FPS... and I've seen some games starting up in the wrong resolution for my monitor, using the hardware blurry linear filter that makes everything look awful, and they still shipped it with that and presumably it was not the end of the world for them. The only way to get that really silky smooth motion is to be able to scroll between pixels. If you scroll between pixels with linear sampling, you open a whole can of worms with the edge opacity issue, which is also a normal result in computer graphics and not a specific Construct 2 issue. It's a decision you have to make as a games designer. You can't always have it all.