[Typo3] Advantage and Disadvantage in keeping 1 only typo3 installation

[Francesco di Francia]

Hi.

I read in documentation and in list that Typo3 can manage more sites 
with a UNIQUE installation and an Unique backend.
Infact in download area there's a "source" package and an installation 
package. The source, i guess, is installed in the unix/linux filesistem 
and it's about 11MB.
So, i suppose that one advantage could be to occupy ONE ONLY TIME 11mb 
of hard disk space.
Which are other advtg of an UNIQUE typo3 installation ? And disadvantage ?
What should i keep attention at if i choose the unique installation ?
Keeping an unique installation improve typo3 perfomance ? Or is only a 
matter of occupy less space ?

Thank in advance,

Francesco