The mechanical tag is pretty useless -- nearly the entire bike falls under this category, so using this tag doesn't really help anyone. There are a few electrical issues (which may go up as electronic drivetrains go up; unfortunately, proprietary-ness will often mean going to a shop for the servicing, and we don't really do electronics of lights and stuff) and aesthetic issues, but they're more the exception than the rule.
Do we need still it?
No - delete it. You're right its about as useless as a tag for "bicycle"
In almost all cases there's a more specific tag -- either a part like gears or brakes, or a process like repair or maintenance. A mechanical tag adds nothing that couldn't be better handled by one or more of these.
There is a tag for disc-brake and one for hydraulic-disc-brake.
So this is one case for a [mechanical] tag or to create a mechanical-disc-brake tag to make this distinction. Perhaps the latter is still better
