Poeple have always complained about Mercurial Demo being a weak set, which is probably because it's MSI is barely notable and not very useful without the Swiftsteps sprite perk. But the shock resistance is good for both PvE and PvP. Mercurial Demo is oftenly used by LD bombers mainly because of the shock resist since people tend to spam Polaris on bombers, but that's all, the MSI isn't exactly it's main selling point. The MSI med isn't that useful and it doesn't give Mercurial Demo users a big movement speed increase to dodge strikers since their speed boost is high enough to let them slash bombers with easy.
Swordsmen and gunners can make use of three bonuses which are ASI, CTR and DMG bonus, while bombs can only make use of two which are CTR and DMG bonus. So why not let bombers make more use of MSI? MSI can help with bombs like the Big Angry Bomb, Electron Vortex and Irontech Destroyer which gives the user a slow walking speed while charging. It also lets bombers dodge more effectively and run away to heal or prepare another strategy.
Mercurial Demo is somewhat weak to qualify as a shadow lair reward so why not buff it a little bit? People have always wondered about how MSI max feels like so why not finally give us that MSI?
No. If you're going to give MSI med per piece to any MSI armor, it's going to be Mercurial, and not Mercurial Demo (and I would add some negative attribute to balance the speed boost). The way that armors are balanced, Mercurial Demo would have an overall +3 per piece without any significant drawbacks.
MSI has not been buffed because it's seen as too gamebreaking, because players are supposed to stick together with similar walking speeds (this would be especially broken with MSI max).
However, I don't care about the "sticking together" part, so much as the "game is broken" part. The main issue that I have with buffing MSI, is that it trivializes everything. With Speed boost orb on MSD medium, I run about as fast as a MSI low knight. Speed Boost increases by 2-3 (assuming a preset boost, and no upward or downward scaling based off of MSI). Thus, MSI max should be about as fast as MSI high or very high with Speed Boost, which is utterly ridiculous to have all the time.
Then on top of that if you get Speed Boost... What's the point in playing the game anymore, practically all enemies will miss and you can run circles around everything (except the fact that you'd probably run too quickly and get hit on the second time around the enemy).
What do you think bombers would be the main people using this Mercurial Demo if buffed thusly? Heck no... an absolute ton of non-bombers would be using it for PVE and PVP.