Tanking to gully- how does this work?

[cwr]

So, if I understand this right.... there seem to be lots of gullys like this one (rectangular one below) where by the tanking/tray is sealed onto the flange around the outside. I understand that if the grouting between tiles fails then the water is contained in the tanking, but it can not drain into the gully over the lip. Unlike in this white mcalpine one ( 2nd image), where the tanking seals onto the round flange. Is it ok for the stray water to be contained and just evaporate out in time, or worth fitting a gully designed to drain the water away?

[Russell griffiths]

All I can say is I fitted the mcalpine one and was reasonably impressed with it.