Personally I'd vote for the first rationale you went with. The boolean operators have short-circuit semantics, which would make for some really gnarly situations if translated into asignment operators. Either you don't make them short-circuit anymore, or you created some weird "optional" assignment operator (do the stuff on the right and assign in the result only if the value on the LHS is already non-zero). Either way you'd create subtle bugs because people would be expecting the other behavior.
