Haha we have been asking this since day one. The answer is never. I don't know why though...
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Probably because people would boost and get crucible/exotic bounties done in one or two rounds... but there are ways around this. For example, a Scrimmage mode that gives you crucible rep/marks but doesn't fulfill bounties or affect character statistics.
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If custom games were implemented they would obviously not provide anything in terms of progression whatsoever.