If a lens doens't come with a hood, I buy one. A lens hood is almost always mandatory - it improves contrast, reduces flare, and provides some protection. Folks often think you're a professional (even if you aren't), and that can make taking photographs just that little bit easier!
That said, this is probably the only lens hood that I'd say is optional, for two reasons.
1) Macro photography means that you usually can't use a lens hood, as it interferes with the flash or generally just gets in the way.
2) An unhooded EF100mm f/2.8 lens probably shows less flare than any other unhooded long lens I've used. It's one of Canon's best non-L series lenses, and image-wise, it holds it's own against L-series glass.