I feel your pain and I think it's a worthy question.
As a consumer it's all about support and the fact is HD DVD doesn't have any support anymore. To stay in it's all the more important to have a backup player to watch the titles you want in case your player ever dies since they aren't making them anymore.
Then if you are a fan of lossless audio and the HD DVD titles you have are now on BD in lossless audio, personally I can see the reason for the upgrade. I did the very same with Bourne and U-571 and the audio upgrade is worth it for me.
For me those are strong considerations as to wheter to keep the HD DVD's you have.
What keeps me in is that I enjoy the HD DVD discs that I have and though i've sold/traded off a number of HD DVD's, of the one's i've kept, I feel as if those versions are superior over what is on the BD equivalent (e.g. Dreamgirls, Training Day, Phantom of the Opera, Shooter) so i'm loathed to give those titles up. Also many of the titles that are on BD will not see a tangible improvement over their HD DVD brethern, particularily those Paramount, Warner titles that share both formats. I'm sure when Universal starts to release on BD those previously released HD DVD titles, many won't see a quality upgrade (outside of lossless audio and that may only be noticable on certain titles).
Price can't be overlooked and with so many titles being discounted and so many selling off their collections, you can get great deals and build up a relatively inexpensive movie library quickly.
I think for now i'm likely to stay in and though I have the BH 200 combo, I think I may down the road try to purchase an hd-a35 that would meet my needs and then upgrade my BD player when necessary.