It was thought at the time that the amount of fuel leaking through that channel in the larger motors was insignificant compared to the amount needed and could easily be compensated for by turning in the high speed needle valve. It is absolutely necessary on the smaller engines as that leakage could be a significant portion of the mix. Some small outboards would run rich with the high speed adjustment screwed all the way in.
If not seeing a gasket their makes you nervous, it does no harm to put one in . . . 😀