That's actually the standard way of dealing with different ratios and pixel densities. Don't get me wrong, I absolutely love construct, but I really think this is a very important missing feature.
Most of the top SDK's handle loading assets this way - Corona/Gideros/Stencyl, even Gamesalad as a resolution independence feature that allows you to load HD assets.
You wouldn't necessarily worry about creating 1.2, 1.5, 1.7x assets etc, but you should definitely worry about loading low res graphics on low powered/lo-res devices and loading hi-res assets on high powered/hi-res devices.
So, having at least 2 sets of assets - one SD and one HD would definitely be the way to go.
Trying to run a game designed for 1920x1080 on a 480x320 Android device might not be optimal as far as speed goes and neither would the opposite, as the graphics would look very blurry/pixelated.
The other bonus is that if you decide to update your apps in the future ( as resolutions get higher ) you can just upload higher resolution graphics with a (for example) — suffix and the engine will take care of the rest ( only use those graphics if the device is a very high definition TV or tablet ).
If you design your graphics as vectors, then exporting different sets of assets is no issue at all :)
I really think that this would a really good feature for Construct 2 to add as it is currently the best/only way to make sure you graphics look good across a range of devices (short of using vector graphics).