The appeal is that they are fun.

I think that people have lost touch with what the whole point of games are: To have fun. Deepness and immersive-ness is not the measure of a game. Don't get me wrong, I like big AAA titles like GTA and Assassins Creed, but I can also appreciate Angry Birds and Bejeweled Blitz because they are a lot of fun.

Final thought: I think that Playstation Now could be an indication of where 'mobile' gaming could be heading, or rather how mobile and traditional gaming are going to collide. You could have the Playstation Now app on your iPhone, you sync a small controller with blue-tooth, and bam there you are on the bus instantly playing Assassins Creed 3.

The answer is pretty straight forward! They are mobile and players hence have the comfort of playing basically anywhere, any time and at a fairly reasonable price. Mobile games are great pass times and they are way cheaper than any sort of titles out there!

It's much more acceptable to play with your phone than with your DS/PSP/Vita... during the class.

I still prefer the traditional handheld consoles tho.

Games are cheap or free and most people already have the platform for other reasons.

As for the appeal and value? I have yet to find a mobile game I would say has any appeal to me.

They are just cheap, that's all really. Shame many went with micro transactions

@OP:

It's cheap, even free, and, you know, mobile.
But in general it's quite bland compared to gaming on portables, its strength is just to be a decent cheap or free extra on phones and tablets. Obviously a strong point for the majority of users, but more dedicated gamers like us will find it acceptable for limited time and will generally want more. For many of us, though, this "more" can be provided by PCs and home consoles, while a phone or tablet could already be enough for mobile/portable gaming if we usually have little time to dedicate to it when we're away from our main gaming machines. So if we usually go by train for long enough trips often enough, we'll possibly want a portable, if we normally drive our cars, or go by crowded buses and undergrounds, often without being able to sit down, mobiles could be enough.