It would seem a bit excessive to me, and i think someone in the discussions i linked to also thought so, but who knows? As long as webcitation.org is only archiving text and not photos/video (i don't know about the latter), given that Terabyte hard disks will become "normal" within the next year or so, it wouldn't put much additional pressure on internet/computer resources to do this (collectively for all wikipedians) for a few hundred million cites and store it on just one single hard disk... Personally, i'll probably just try and see. You can probably make a mental note (or note on your user page) which journals have a habit of being volatile. Online newspapers from places with relatively weak internet access and less developed internet/GNU/linux communities (e.g. Iraq!) probably might be volatile in the sense that after a few years they will change software/hosting/URLs or go offline for whatever reason. Another point is that webcitation.org probably doesn't (yet) have a long term reputation. archive.org and arxiv.org have been around for a long time, arxiv.org has very solid institutional backing and a huge amount of mirroring. In any case, i don't see why anyone could complain if you archived every cited reference.