Soyuz in Shuttle?

The reason that the mission to service
Hubble has been cancelled is that they
can't guarantee the safety of the crew
for trips that are not close to the Space
Station (which can serve as a refuge).

However, why not carry a Soyuz capsule
in the Shuttle cargo bay? Then if there
is a problem with Shuttle reentry, the
Soyuz can be used as a lifeboat.