I can understand what you are saying, but I think it is a little bit of an over-raection.
Pages can open the proprietary format; if you can save a copy of the document, you can also save a backup of the application. Twenty five year old software can still be run (in emulation) on at least Mountain Lion, so I don't expect to outlive my ability to run Pages if I have to. And Pages is a personal format; if you are producing documents for others in Pages, you would normally produce output in pdf or print.
Many of the documents I produce in Pages are temporary items that I only want to have around long enough to produce a pdf or printout; I don't even want to save them to disk. And very few of the documents I create are anything that I would want to open three or four years down the road.
If you are producing documents that you want to be accessible to posterity, your advice is on target. In that case, though, I think the proprietary aspect is irrelevant. That sort of information should always be preserved in multiple formats, on multiple kinds of media, and in multiple locations.