Tony
It's been my understanding ever since Flex was introduced as a separate product that needed to be purchased, that if we wanted to use Flex, we had to accept that Plexus (or as it was then known -- the IO Plugin) was an integral element for Flex.
Put simply, if you wanted to use Flex in your development work, your clients, like it or not, had to download and install Plexus. My experience then was that some clients were happy to do this, others refused.
About 7-8 years ago I had significant correspondence with the person who was at that time responsible for much of the development of Flex. Again, to put it simply, he explained that many of the functions built into Flex depended on the support of features built into the IO Plugin, therefore no plugin, no Flex.
My experience was that too many clients refused to download and install the plugin. This meant that I could no longer consider Flex as a useful development tool. Because of this, I stopped using Flex about 2002 -- I think.
For me, continued use of Flex just wasn't worth the trouble, but other people on these forums have had more success with getting their clients to download the plugin, and continue to produce publications based on Flex.
If you do a search through the old forum, you might find discussions that will interest you.
It would be great to see an Opus adjunct that can produce Flash pubs without the plugin, but I'm not holding my breath