Can't specify player coordinates with /summon via command block

More of a feature request than a bug.

The /summon command only allows for exact x y z coordinates or relative ones using ~ ~ ~. But doesn't allow for @p,@r,@a. A player can issue the /summon command to create a mob near them, but a command block can only summon a mob relative to itself or at exact coordinates.

The following commands issued by a command block summon a sheep at the command block's coordinates:
/summon Sheep @p
/summon Sheep @p[name=playername]

I would expect that the above commands would spawn the sheep relative to a player rather than relative to the command block itself.

Some uses for this:

• Summoning "Wandering Monsters" in an adventure map

• A "create jail" spell using FallingSand to make blocks of iron bars around a player, not just when they step on an exact spot

• Dropping a chest of goodies at the player's feet