And even then they cannot be online full-time if they're from outside systems.
It's not just that.
For starters, a patch of a very active "foreign" channel that's constantly active can effectively bypass the talkgroup priority settings, thus leading to more busy signals for critical users.
In addition, there are a variety of issues with patches of any sort; for example, a user on the patched system could be suffering a stuck mike (or a stolen radio, or junior playing with daddy's radio, etc). Also, improperly set up patches can hinder interoperability rather than help it, as the cruddy audio or ping-pong effect can really screw things up.
In any case, patches have the potential to cause MARCS to lose control over interference; the offending radio on the patched system cannot be disabled by MARCS, only the radio making the patch on the MARCS side can be disabled. If they have to go that route, the patch is no longer of any use unless the patch radio is reactivated.
One should never rely on a patch for coverage enhancement or day-to-day operations; a patch should only be used only when absolutely necessary, and then for only as long as necessary.