We act as our own general contractor for all our renos, and do most of our handyman work ourselves. Plumbing/electrical type stuff we leave to people who are certified to do it.
I don't think it hurts to do the work yourself, but it's not necessary either. One thing I find is when I've done something myself before, I know the right questions to ask and how long things should take when I'm getting quotes/bids from subcontractors to come and do an individual job. Someone without that experience could just get a few bids from general contractors, accept the additional markup, and only buy properties that work within those criteria. It's not a better or worse plan, just different.
Regards,
Michael