It's been in the game for years now, and every year I hate it more. Here's why.
1. ** It's unnecessary, we're already limited with how many chests we can get. **
We're already limited to 1 chest per week, I don't really see why we need to add a further restriction on top of that. Furthermore, Keys are a limited resource as well, and a clearly intended to be the true limiting factor (demonstrated by event shop selling keys, but not chests)
2. ** It penalizes solo players.**
If you're playing in a premade party, it's easier to get chests than playing solo. If you play solo you have to play a champion you haven't already gotten a chest on, and then get an S- or higher. If you're in a premade however, you just need to play a champion that you're eligible to get a chest on and if one person in your party pops off, you get a chest. This punishes you for playing solo, by making the process of unlocking chests more grindy and frustrating.
3. **It encourages a style of play that may be detrimental to your team as a whole**
Let's say I need a chest, or 5. So I go into champion select and pick a champion that I am eligible to get a chest on. From this point forward, my goal shifts, at least partially, from simply winning, to getting an S- or higher ranking. What this means is now I'm more protective of my stats that influence the champion mastery grading system. Let's say I have an opportunity to initiate a team fight. Now in the process, I will almost certainly die, but my team will win the fight. Should I go for it? Well if my goal is to win the game, then absolutely. But if I'm playing for an S ranking then.... eh..... it depends. This encourages people to become "KDA players". Another example, let's say I have someone on my team that's a bit behind, or maybe they're not behind but regardless, a really large creep wave would be way more useful on them than it would be on me. Buuuut, having high CS is really important for the grading system, so instead I greed out and take the minion wave. When you're in game, the ultimate goal should be, take the enemy nexus, and not to worry about stats.
4. **Most importantly, it makes me feel like I'm being penalized for playing my favorite champions**
I've always kept a pretty small champion pool. I might change which champions are in that pool every so often, but at almost every point I'll only have maybe 5-10 champions I play regularly. Right now I mostly play Warwick, Kayn, Veigar, Ezreal, and Blitzcrank. When I've got 5 keys saved, with 4 chests available and I go into champ select, I have to decide between picking something I can get a chest on, and picking something I actually want to play. I feel like if I pick one of my favorite champions, I'm being penalized, I can't get a new chest, and those keys I have will just be covered in dust, and I'm missing out on free stuff.
I understand that Riot want's to encourage players to try a variety of champions and to try playing in premades more often. But I feel like this current system not only doesn't do a good job of that (it's not enough to try a champion, you have to master it), but that it does more harm than good.