Apparently it's not on vB by default because it adds a query for ever user.

As Spank already replied, this is left out of the default vBulletin because it adds queries to every single page load. Adding this can have a negative impact on the performance of your board.

You choose the permissions that show up in the ModCP using the Moderator Manager section of your ACP. You can grant ability to ban, edit signatures, prune, move ect ect.

But how much more strenuous is the if condition="can_moderate()" query than the if condition="$show['modcplink']", since this replaces the latter with the former?

I have all my mods set as super mods anyways.
If I had a board setup that had mods that could only mod selected forums, I don't know if I'd want them to have access to the modcp.

just my 2 cents :)

Oh. http://www.web-city.it/forum/images/smilies/fermosi.gif
I didn't know this. Thank you, Marco.

They already do, it just doesnt show a link.

They can access it by entering the URL into their browsers manually.

Which is very unhandy. But they can go from a member profile to the modcp, which is what my mods did until I added a Staff tools dropdown in navbar with a link to modcp.
It may add queries, but why cause more work for your staff? The adminCP link in the footer ads just as many queries and is used by less people (if you have less admins than moderators)

The AdminCP link does not add any queries. ;)

AdminCP access is based on a usergroup setting, and already loaded into memory.

ModCP access for Super Moderators, see above.

ModCP access for regular moderators is however not a usergroup function and need to be read from the database on each pageload.

What about this template modification ?
I don't think it will add a query :)