It does seem astonishing to see the
Cameos coming out. I mean, those things are nine times obsoleted. Remember when the
Shards of Alara Obelisks came out?
Bant Obelisk 
and friends? Those things were awful even by standards of the time: they were inferior to printed cards such as
Darksteel Ingot 
,
Spectral Searchlight 
and
Coalition Relic 
. Since then there's been
Manalith 
and
Chromatic Lantern 
to add to the number of cards strictly better than all five Obelisks. And yet, the Cameos are
even worse than the Obelisks.
Just... wow. Why would you ever have a Cameo in a modern deck? There's
Selesnya Signet 
and
Talisman of Unity 
, as well as
Prismatic Lens 
(which is far better than
Mana Prism 
) and so on. If you've still got Mana Prism sitting in a deck in 2012, I don't know what to say.
(Unless it's for a non-gameplay reason, of course. I'm completely fine with keeping Mana Prism around because you have a deck themed around prisms, or girls with blue hair, or a Margaret Organ-Kean theme deck. But there's really no
gameplay-wise excuse to have Mana Prism unless you're already packing the full set of
Darksteel Ingot ,
Spectral Searchlight ,
Manalith ,
Coalition Relic ,
Scuttlemutt 
,
Alloy Myr 
, and still want more 3-drop mana rocks. And I think that's unlikely.)
I suppose it could have been worse: it could have been a
Celestial Prism 
...
