for wide range of application use Letterbox scale
I already use letterbox scale, but I'd rather not have black bars on my games!
One of the reason I heard was because It'll be ugly (which is mostly true, with a lot of different aspect ratios, the stretch can be really annoying), apart from that I don't know if there is another reason.
Indeed, I can see how that would be an issue, especially for a mobile-designed game displayed on a desktop, but what about games that will be targeted only for mobile devices?
My first released game was created using another engine and the scaling works fantastically on the different devices I tested (Phones + Tablets).