Partner adds an extra wrinkle to the package, effectively allowing you to run two copies of a commander from among the options with partner
So, if you can't keep it alive or get through chump blockers, you can end up with a four-mana 4/3 that isn't doing much
Its a powerful stax piece that forces your opponents to pay more mana or drown in card advantage as you draw more and more spells
To start, it has more cost reduction options (though I consider artifact the most useful)
They all offer something a little extra, and Conspire is going to be worth it most of the time if your deck goes wide enough
Fortunately, there are hundreds of them, so finding enough to play even in a singleton format such as Commander won't prove too difficult