I've already fully rendered a chapter in HD in VMS10 and there are a whole bunch of pre-rendered clips taking up loads of space on my media hard drive.
Should I save these pre-rendered clips for posterity (as in back-up files) or are they pretty much not necessary now ?
Obviously, it would be nice to archive ALL elements of any project, if you have enough cheap archive storage space available.
Only YOU can guestimate how likely you or others are to "re-work" or to recover your original, project-related elements.
If you are acquiring content at higher rez than your final render, then saving all ingest and elements makes more sense, as you (or others) may wish to render at highest quality, later. But, if your final output quality, color correction, lighting, audio, etc., is all as "good" as it is ever going to be ... If you are fully satisfied that this is the best version that you or your heirs will ever "cut" ... then just send a full copy of your final project to as many friends and family members as might archive a copy of it, for posterity, "off-site".
Cost per GB of magnetic HDD media is REALLY getting very low ...
... A SAMSUNG SpinPoint F3 1TB HDD or Seagate 7200.12 (1TB) HDD cost under $80 and often go on sale for $70 ...
... At ~eight cents, per Gigabyte ... That's about $01.25 per hour of highest quality HDV/AVCHD at 1080P/30 25Mbits/sec.
The qualitative "worth" and future value, of your project elements, is something that only you can decide.