Since you own a 3090 FTW3 I would recommend keeping the other card as a backup.

I bought a 3080ti FTW3 as a backup to my 3090.  My 3090 is a Zotac, so obviously I had more motivation but other things can happen to a card that wouldn't be covered by the warranty.  When I got a chance to get the 3080ti, I jumped on it so I wouldn't have to worry about being without a good card.  The 3080ti has a good home in my Plex machine and powers MadVR for my projector.

Yes always good to have a back up....but you certainly dont need a top end card for back up....I consider back-up cards also for testing purposes and for new builds if you want to recycle a really good top end card.
 
I have a 1080Ti SC Black and a couple of old GTX 560Ti's for the purposes above.
Also have almost 2 complete systems....
- X79 Rampage IV-Extreme / 3930K / 16G Dominator DDR3 /  Evga 850W
When I get my DDR5 I'll have spare Maximus XI-E / 9900K / 32G Dominator DDR4 / AX 1200i
- spare mouse, spare keyboard, spare D5 Vario pump, spare fans, spare "cheap" 1080 monitor
- also an SSD hoarder ;)
 
So handy having a spare system and a few spare parts ;)

nomoss
I had the 3090 FTW when my Kingpin notify came.  Tried to sell it and then talked the guy who responded out of buying it.  

As a car salesman, I know I can make bank on selling it.  Buy my only backup is a 1070.  Decided to keep the 3090 just in case because I will NOT be going back down from 4K, dagnabit!

While I was waiting on my 3080Ti RMA I threw an EK Block on my old 1080Ti SC Black as I was having CP77 withdrawals.....monitor is a Dell Alienware 3440*1440.....
With the 3080Ti everything cranked up excepy "psycho mode" around 80 FPS
 
With 1080Ti had to drop everything down to a mix of Low / Medium (mostly low) obviously no RT...but you know I was getting 55-70 FPS and visually it still looked pretty darn good !!!
Absolutely playeable!!
Bloody old 1080Ti SC Black w a Block on it still rocks its age...in my large loop it never saw more than 37c ....

keeping a backup gpu is a good idea now days since gpus is difficult to get, I'm keeping my 1080 Ti SC2 when I get the 3080 Ti or rtx 3000 refresh or RTX 4000 whichever comes first 

Backups seem prudent in this environment.

I have a backup pc w/card + another backup card too. I can't risk something happening because I work from home a lot.

I keep a very old graphics card for emergencies. 

Yes,keeping a 3070 as a backup to my 3080.You never know Both are ftw3 ultra's

 
Backup card - yes. 3080 Ti as a backup card - nope, at least not at present. I'd sell the 3080 Ti while the market is strong, and buy a used lesser card, maybe a 2070, just for emergencies. 3080 Ti is an expensive and powerful card, seems to be a waste just to have it sitting there doing nothing but losing value.
 
I'd make sure the 3090 is solid as a rock first though.
 

For a 3090 , I would if I could, although I think they are getting it sorted out, the spikes in the last firmware seem much more compressed

If this turns into a 2 page thread, it's been a slow day.
If I felt I needed a backup, I would buy something lower and cheaper and sell the 3080 Ti. Actually, to be honest, I would sell the 3090 due to its track record, and keep the 3080 Ti. Actually to be honest, I would never buy the 3080 Ti or 3090 in the first place.

Salt level update:

talon951
I bought a 3080ti FTW3 as a backup to my 3090.  My 3090 is a Zotac, so obviously I had more motivation but other things can happen to a card that wouldn't be covered by the warranty.  When I got a chance to get the 3080ti, I jumped on it so I wouldn't have to worry about being without a good card.  The 3080ti has a good home in my Plex machine and powers MadVR for my projector.

are you kidding i sold my zotac 3080 and it ran perfect for a evga 3090 worst trade off i ever made,i hate this card
its a wonky pile.i still have my evga 1080ti last good card evga built imo.i'd bet you money your 3080ti tanks first !!!

celcius
Thanks guys, I'll just keep both cards.

Market isn't going to fall out on used prices anytime soon.  I kept mine because if I use the Classified tool/bios on my Kingpin, it's no longer under warranty.