Yes, I know, strange isn't it? I think that because I always send the passport, or a certified copy of it, they just don't care about the birth certificate. I guess if you were sending other evidence of the sponsor's residence in the UK then the passport would be essential.
They only started asking for birth certificates a few months ago, so it may just be that the consulates haven't caught up. However, it should be noted that these are just guidelines, and if the requirements as per the rules are met without the birth certificate being supplied, it would be unlawful for them to refuse the application.
I stand by my initial statement - if it is a major hassle to find it or get hold of a copy, don't worry about it.
Vicky