I wouldn't say recommended as such, not in the same way they might recommend you change the filter, but certainly it was mentioned that the bag could be emptied once and re-used. That is to say the bag could be used twice in total. In reality I found some people only used a reusable bag once, whereas others reused a single bag many times over. I don't just mean on a 500 cleaner, I mean any reusable bag. It all comes down to a changing attitude towards paper bags. Hoover were one of the first to use a paper bag and almost all of their cleaners with paper bags could only be used with such, unlike many other brands and models where the paper bag, if available, was entirely optional. Some people say that the use of a paper bag was considered to be somewhat decadent.
As time progressed, more and more cleaners came with paper bags only, and then towards the end of the 1970's the fashion for being able to reuse a dust bag was becoming more popular, having always been able to do so on most Hoover cleaners already. It was during the early 1990's that the bag slides & reusable dust bags went out of fashion, as manufacturers wanted people to throw away a full bag as they said it was more hygienic and made the cleaner work more efficiently.