This is a very important question. I asked it to myself a lot of times when HTML5 mobile gaming started to become popular. I was just thinking it was a waste of time to simply care about the different screen resolutions and aspect ratios, so my first HTML5 game was made to perfectly fit my iPad 2 tablet.

When I finally showed it to sponsors, most of them said something like "Hey, I like the game, but unfortunately it does not look that good on my iPhone". "Don't worry", I said, "you'll get the game optimized for iPad and iPhone". Unfortunately, it did not look that good on the Galaxy Note. Neither did it on the Samsung S4.

You can imagine the rest of this story. I found myself almost rewriting the game with a series of if.. then.. else loops, trying to make it look good on any device.

This is why you should make a cross-platform game: To code once and rule them all. Focus on game development and let a framework do the dirty work for you.