The cassette plug is a very useful bit of kit under two or three or four or five circumstances and I am very pleased to hear that it remains a standard part:
1. removing the drive and replacing it with a plug gives you noticeable extra speed when sailing and makes the boat respond differently (and better)
2. the same applies to paddling - if you have to paddle any distance you should remove the drive and replace it with the plug - the drag of the boat through the will be reduced considerably which is of great interest if you are trying to propel it with a paddle!
3. if you have a double and are taking littlies out or are transporting e.g. camping gear in one or other cockpit it is very useful to replace the drive with the plug; you don't need the drive and a big hole in the floor could be an issue for little ones. The amount of water sloshing about in the cockpit will also be dramatically reduced.
4. some people have filled their drive plugs with ballast for use on a double.
5. if you have to pull the drive at sea to repair it, it is useful to put the plug into the hole so that if you drop a tool or a part there is less chance that it will disappear through the hole in the floor! (can anyone detect the experienced tone in which these last words are imparted ?