I just saw this old thread, but I think that as a GI player AND as a GI premium collector, I can answer this question.
This card somehow basically screams "I'm so common!!" to me. Why was it rare in the first place?
Yes, Mechanism seems common.... now. The reason why Mechanism was a rare, and the reason it seems common now has a lot to do with Omnitron, and Omnimind as well. Omnitron is one of the most popular triple-GI strategies (if not THE most popular), and Mechanism is a card that has always been VERY useful to work your way up to deploying Omnitron. It is cheap, it gives you a nice +1/+1 buff (it's not much, but sometimes it can make a difference between saving a creature from dying to a spell, or effect like poison 1 for example), but the most important thing was the first one, it is cheap, as cheap as it gets for artifacts, and that means a lot when trying to get Omnitron into play.
That made Mechanism a good card, a VERY good card in fact, most artifacts you deploy don't always get used every turn, but Mechanism gives you a passive buff, you don't even have to pay to activate it. All those reasons made the card be a rare, and I think it was reasonable.
However, things changed when Omnimind was added to the game. Suddenly everyone could have mechanims in their support zone. Sure, they cost most if you create them through Omnimind, but Omnimind made (makes) more sense in many cases, it lowers the cost of artificial creatures, it can create Mechanisms, Unending Drones, and buff artificials too. In comparison, Mechanism suddenly seemed like it wasn't powerful enough to be considered a rare to some, but it still kept being a rare, because you couldn't deny that it still made things extremely cheap when trying to deploy Omnitron, and that's a huge advantage to any GI player.
And then... came draft balance... which leads me to what Zekses said:
I could not care less about mechanism's rarity. It's the very concept of basing card rarity on Draft that irks me.
Yodomare seem to balance draft using rarity of cards. Mechanism was showing up in the 'rare slot' when drafting decks, and that meant that the slot was being wasted on a cheap artifact that works brilliantly in an Omnitron deck... but you're not building an Omnitron deck in draft, are you? So why should Mechanism take the rare slot when you could be drafting something more rare-worthy like Unstable Bomb-bot, or Lanstead for example. So they decided to change Mechanism rarity to common...
This brough a ton of negative feedback, not only because they were using the rarity system to solve balancing issues in a format that should rely on something different to balance itself (this is still true to this day), but also because traders and collectors had been trading Mechanisms for years (literaly) as a rare. Heck, I myself have traded and bought platfoil Mechanisms, and every time I traded/payed for them as 'rares'.
All that negative feedback ended up making Yodomare change its rarity from common to uncommon. It's something... but a lot of players (including myself) ended up taking things like this like a slap in the face, cause after +2 years suddenly this rare wasn't a rare anymore, and in a 'trading' card game, changing card rarities means chaging card values, and changing card values after 2 years.... well, that's just horribly wrong, and very unprofessional if you ask me. Why do I say unprofessional? Because if you're going to call your game a 'trading' card game (among other features) then you need to stick to what you publish unless there's a VERY good reason to change that... and balancing draft through rarity wasn't one, wasn't even a decent reason because as I said before, draft should be balanced differently, a card rarity shouldn't have to be forced just because of one format.
Anyway, in the end Mechanism ended up becoming an uncommon, which is the current state of the card. Personally I'd like it to go to where it was before, a rare, but I know that isn't going to happen so why bother... >.>