Slightly less stress so maybe a longer life, though dead RAM is rare. Especially if you can also drop the voltage a bit and have it still work. If it is something that warrants some active cooling or a giant heatsink on top of what might already be on there then that too.
For that kind of underclock you might be able to drop the timings a bit more as well. Timings tend not to mean much though outside of tests specifically contrived to make them work and even then mostly when comparing them to multiple times slower timing values.
To that end if you want to sell it and you can get something for it (possibly before buying some equal capacity but slower stuff which is going cheap) then might as well.
I don't think it will end up as some kind of even more pricey thing in the future (seen the price of the very last AGP graphics cards* over the last few years after everybody found that new games often suck and there is a lot of good stuff** from DOS through end of XP which often works best on AGP vintage systems?) but if it is gamer grade stuff then that might change things a tiny bit as people do often like to build would have been $5000 at the time but today should be pocket change machines. I also don't know what will happen in server world (when DDR2 was ridiculous thanks to everybody seeking a quick upgrade to hold them for a while the DDR2 market got flooded with ECC memory from servers, which requires a compatible board but many were** and the server motherboards and processors were also going along with it and by this time had gained PCIe x16 slots, and I could see something similar happening there).
*this was well into the last of directX 9 at this point, if not later for some (though maybe some of the more minor players) and plenty of people scoffed at the time. One technically retro boom later...
**see pretty much any of the slight edgier PC gaming channels, and if they are not playing vintage then it usually could be mistaken as such.
*** as ever I do seriously miss the thing where motherboard makers would take their server boards, maybe strip out dual power supply input and throw that onto the market as a gaming/high end board. So much better than OMG we stuck a few pence of colour changing LEDs I now have to colour in with permanent marker, oh and good luck with having nice onboard features and lots of slots unless you pay us the serious money.
I appear to be ranting in asterisks so I will tie it off there. Sell if you have something of suitable capacity to replace it with (which is prices here are anything to buy should be easy). At very best you might look back in 10 years and find you could have made a tiny bit more, however that would be in 10 years.
