Since USE=multislot is invalid per PMS and breaks caches with a reasonable package manager, please mask it. If users really want it, they can still unmask it while normal users wouldn't enable it thoughtlessly.
9 years strong and people system's still haven't blown up. don't see the point.
(In reply to SpanKY from comment #1) > 9 years strong and people system's still haven't blown up. don't see the > point. Mostly because portage worked this around which is broken by design. Other PMs will hit cache invalidation which doesn't involve blowing up stuff but degrading performance.
(In reply to Michał Górny from comment #2) i'm more than aware of the history. see bug 174407 for the details.