Why of course there are benefits in paying!
You can invest all your Influence Points on runes, while affording champions with hard, cold cash.
You know you are supposed to pick your rune page according to what opponents you are facing, right? It is usually not optimal to have only one set of runes, or one single strategy with your main champion. To be flexible, you really need lots of Tier 3 runes.
Unless, of course, you have already owned all runes. In such case, my advice to you is to get a real life
The patching mechanism is plain stupid. If I need to install the game and download the latest patcher from the website, why can't I just get the full up-to-date version? Are they really so lazy that they won't re-compile the installer after an update? So I need to hack the patcher just to get more patches? What is Riot thinking? We don't need to look further than Fortress Forever for an example of how it can be done right.
$20 sounds about right for a game in beta stage, such as LoL. No more than that. I will pay them $3 more if they set up a decent server.