In my opinion, the point of the time-wasting game is exactly that - to pass the time whilst waiting for the bus, or whilst on the train on the way to work or school. Sure, it's different from what people were doing on their commute a few years ago, such as reading the paper or a book, chatting to other workmates/commuters or staring out the window watching the world go by. But now, people can spend that 20 minute journey (or however long) attacking pigs with flying birds, or feeding candy to a little green monster (at 7.30am?? Bit early for sweets, isn't it?!). Yes, the storylines are dreadful, if the game even has one! But the point of one of these games is being very stop-start - you can put it down to work throughout the day, then on your way home, you can pick it straight back up again, with very quick loading times. You don't have to remind yourself where in the story you are, it doesn't really matter! The fact that these games are available on mobile phones, MP3 players and tablets, many of which people have on themselves at most, if not all, times, means that people can play them wherever, whenever. I'm going to the dentist later today, and no doubt, whilst I wait to go through to the Chair of Pain (my dislike of dentists is another post altogether!), I'm sure I'll probably be playing some kind of game on my phone. It's significantly easier playing games on my phone than hauling around a console and TV (or PC) to play on!
Also, I think another attraction of these short-term games is that most of them are very cheap or free, whilst the Mass Effect trilogy is about £40 total. For that amount of money, you could get 50 or so games priced at 69p, or 20 games priced at £1.99. The point is that you can get a significantly wider scope of games for a similar amount of money, if you so choose. Yes, the games aren't going to be as deep or as well thought-through as "proper" games, like Mass Effect, but it means that throughout one day, if ever you get bored of one game, it's very easy to switch to another.
I do like these mobile games. I'm especially a fan of puzzle-type games, whether that be Sudoku, Flow (in which you have to draw different coloured lines to different coloured dots), or Unblock Me, (where you have to free a red wooden block from a congested board of other wooden blocks). I find that they do pass the time on the train, or wherever. Maybe this time can be better spent doing other things, like reading, or studying (probably need to do more of that anyway...), but playing on my phone is so easy. That's why mobile gaming on cheap, some say "tacky" games is here to stay. Although, something is true with Matt's final point - watching people throw live birds in to other people's faces would certainly liven up the commute!