Rhumb lines may be useful for tiny vessels with only a compass and a map on board, but any commercial vessel, whether it is on the water or in the air, will follow the great circle route. It's the shortest and fastest way to go and it uses the least amount of fuel. Following rhumb lines just because they happen to be straight lines on Mercator projected maps would be silly.

It's not the 1800's any more. We have computers and stuff, we don't have to rely on techniques developed for a time when your navigational tools consisted of a map, a compass and a sextant.

Computers don't care about your maps and will plot a great circle route just fine. However, in the event of system failure you want a real backup. Which is why the navy still teaches people how to use a sextant. http://www.npr.org/2016/02/22/467210492/u-s-navy-brings-back...

Note: The actual deviation from a great circle route is generally fairly minimal in practice.

PS: GPS is just another system that really can fail.

The passage of time doesn't diminish the usefulness of local navigation techniques...

a Mercator projection will give you a bearing to steer to arrive at your destination, which can be followed by eye, autopilot, etc etc.

Calculating a great circle arc and plotting it, following a bearing which may only change by a degree or two but isn't constant, is an unnecessary complication on short routes.

Even commercial vessels will use those old techniques if they don't need to consider the longer range effects. Error correction in the human process is always a consideration.

(Not even talking about data entry errors in GPS systems, but that Malaysia - Melbourne story was Funny.)