Trusted and Untrusted X

 Since version 3.8 OpenSSH has given the user the "-Y" option to enable
trusted X forwarding.  After reading the SSH documentation, the new SSH
O'reilly book, and the X man pages I am still confused as to what this is
actually giving the user or why they would let the user select this option.

   My understanding so far is that normally X forwarding is defaulted to
untrusted.  This limits the capabilities of the user so that they cannot
easily gather information from other windows handled by the X server (i.e.
keystroke monitoring, etc.).  By using the "-Y" option the user is now able
to access things normally protected by the X server.  This is notably
necessary to use Perl/TK over these connections.  I guess this is because
Perl/TK is making calls that are normally protected by the X server.

   My question is this.  Is my description accurate?  Also, why would they
let the clientside handle this and not provide an option on the serverside
to control the access privileges of the incoming users?  Are there other
regular instances where trusted X is necessary?

Thank you in advance,
Don