Let's talk about the rune Celerity.
Celerity is a rune that grants 3% bonus MS and also any bonus MS you get (boots, items, abilities) is converted through a small percentage into adaptive AD or AP. So, for example, Pantheon who has 355 MS would get a slightly bigger bonus to his MS than a Janna at 315 MS, therefore a bigger adaptive outcome as well.
Currently on the PBE, they are changing this rune so that it grants 1.5% bonus MS, but doubling the % adaptive stats it grants, which lets me wonder:
WHAT'S THE REASON BEHIND THIS RUNE? WHAT IS IT SUPPOSED TO ACCOMPLISH?
One would assume that it would be good for champion who have a lot of MS through their abilities. Master Yi R, Viktor Q, Sivir passive and R are good examples. These champions gain a small boost to their AD or AP and that's fine. The thing is, the MS stat is generally hard to manage. It's hard to buy items to increase or decrease it, apart from boots and it's a very important stat to have, which is why you see a lot of junglers picking it to navigate through the jungle, alongside Waterwalking. Also, it's a go-to choice for champions who don't have a good secondary tree. For example, most ADCs go for Fleet Footwork and then struggle to find a good secondary tree, often defaulting into sorcery to pick up Celerity and Gathering Storm, over anything else, just because the flat MS it provides is important.
With the potential change coming up, Riot is apparently trying to make this rune more about champions with MS boosts rather than a mobility rune. Is this correct? Maybe. I'm not sure though. I'm not sure we should give a lot of bonuses for champions who are already having access to massively increase one of the most important stats in the game. Catching champions like Singed, Hecarim, Zilean is already hard enough because of their MS and now these champions get rewarded by getting quite a big boost to their AD or AP as well.
Honestly, I like how the rune currently is. It's a good choice to pick up to get some mobility for yourself, especially if you have nothing else to default to. It's also good for champions with some MS buff as well with the current stats. Honestly, no reason to change it.

runs sorcery.