Why is a single join on JoinMarket not that useful?

[markcolls]

The recommendations from all the JoinMarket people is that a single join doesn't really improve privacy that much and that I should always do a tumble schedule (multiple coinjoins) Why is this? When I look at the transaction of a single coin join, the same amount has been sent to a 7 people. Any of those 7 could be me, so I don't get how a single coin join is useless. Could someone please explain.