This feature has been a resounding success. It allows high ranked players to prevent toxic meta picks from becoming overbearing. It allows low ranks to mold the casual experience to their liking. Most players ban for casual purposes. The only reason you would ban a hero like Sombra or Mercy is because they make the game anti fun, not for competitive reasons. These heroes are clearly not over powered or even strong, one could make the argument they are quite weak and need buffs. However they are hated because they ruin the casual aspect of the game.
What is quick play if not a casual experience? It makes just as much sense, maybe even more, to bring hero bans into the casual playlist to create a more casual experience. The only counter argument against this is by not allowing one tricks to play certain heroes it ruins the casual aspect of the game. Again, many hero bans especially in low ranks are done for casual purposes, not competitive ones. I’ll point you to several threads documenting hero bans.
In this post out of a sample of 51 matches the top 5 bans were Mercy, Sombra, Zarya, Sojourn and Ball. Out of these you can argue only one, maybe two are meta picks. Mercy, Sombra and Ball are banned due to being antifun heroes.
This post has a sample of 20 matches with popular bans being Sombra, Ana, Ball, Sojourn, Zarya and Torb. Again we see a similar pattern of heroes with only a couple who are actually considered op.
This reddit post has a sample of 25 games. The most banned heroes being Widow, Mercy, Doom, Ball and Sombra.
https://www.reddit.com/r/Overwatch/comments/1kicdyx/hero_ban_stats_after_100_ranked_matches/
Here’s a sample of 100 matches. Most banned heroes are Sombra, Mercy, Sojourn, Widow and Cass.
We see a clear trend based on the data the community has been able to compile. Hero bans are not exclusively strong/meta defining heroes. They tend to be heroes the community deems to be anti fun or an impediment to the casual experience. Being that quick play is a mode in part made to act as a more casual version of ranked it makes sense to allow players to curate the hero pool to fit into that casual form.