A manual swith or a relay with a manual control would be much better.
Maybe, but it seems to be a case of "if it ain't broke, don't fix it." In the one case we experienced (this was back when the boat had two 8Ds) the problem was exactly as you describe-- a shorted cell. The relay did its job and both engines turned over on their starters as though there was no battery problem at all. That is to say they spun fast and fired almost instantly just like they always do.
That's the only result we're interested in. There are probably a hundred ways our boat could be set up electrically. The way GB chose to do it in 1973 has demonstrated decisively to work as advertised so we see no case to be made for changing it. If we were starting from scratch there are undoubtedly new technologies that would be good to incorporate. But we aren't starting from scratch.