@Cheryl P. - I think that it really depends on the individual investor and the actual cost to rehab a project or what actually needs to be done. Let me explain -
If the investor, is not a hands on kind of person (i.e. not skilled in home repair at all) or simply doesn't want to do it, then of course he'd hire somebody to do it - increasing his overall costs of the project, but might be cheaper than utilizing his time to do it or learn to do it. Depending on the size & scope of work, the investor might be better served finding other investments than painting a room or putting up a ceiling fan in one of his projects.
Now, if the project or scope of work really needs a qualified, licensed contractor (HVAC, electrical or plumbing come to mind), then I believe it's a contractor all the way, UNLESS the investor himself is from that industry and qualified to make the repairs himself. Like a licensed plumber/electrician turned investor -
I myself, an a real estate agent. So I prefer to do all the acquisition & resell myself, but will sub out almost everything else - because that's where my time is most valuable.
What I'm really talking about is the COST of the investors time...is the investor better served making physical labor repairs or finding other investments? - sometimes they enjoy making repairs and have the time, after hours so then it's just preference. However, I am NOT a fan of investors doing all the work, when they are NOT qualified to make the repair, and do a terrible job at it or cover up terrible work - then it's just about the $$ saved, and usually ends up badly for the buyer at a later date.
just my opinion...