To be honest, I hadn't considered that the default key was selected by GnuPG rather than Enigmail, so I think you are correct in saying that the Enigmail team shouldn't change that default setting.
It does strike me as strange, however, that GnuPG defaults to a standard that has been proven to have a potential flaw. I'm aware that the 1:9,223,372,036,854,775,808 (1:2^63) chance of the key being exploited is remote to say the least, but when I discovered the flaw in the keys, I decided to revoke my original key pairs and create a new set of RSA keys for use with SHA256. If I had known about this flaw in the outset, or if the default key settings were different, obviously I wouldn't have had to do this, but I do agree that we should follow GnuPG on this.
I've heard on the grapevine (well, the GnuPG Users list at any rate) that there's a new version of GnuPG 1.4.3 in the pipeline. Possibly a feature request to be made.
I suppose another suggestion would be for the Enigmail website team to include a link to the wiki page that explains the different hash algorithms so that users have the information they need to choose their method of key generation.
Mozilla Thunderbird 2.0, Enigmail 0.95.5, GnuPG 1.4.8-MobilityEmail
OpenPGP Key ID 0x37858A47