Why hire a 'larger' firm? Well, my experiences this past year as a customer might shed some light on the topic.
First off, when I call I need someone to answer the phone. It need not be the actual guy who will do the work - just someone to take my info, and give me an idea when to expect a call-back.
Then, I Do have a scheduler. One guy can only be in one place at a time; while I'm usually able to accomodate, I need a reasonable response. Waiting three months for a service call just won't do.
I'd like the guy who does show up to actually spend his time working - rather than running back and forth to the shop / parts house, on the phone, pulling permits, etc. While these are all necessary parts of running a business, they're parts I'd just as soon not see.
It means something to me that the guy look like he's a pro. That means no torn jeans, a real work truck with real signage, and some tools & materials. It means some proper paperwork and -at a minimum- a business card.
I also realize that some jobs benefit from additional helpers, and some rental equipment. I like seeing work done promptly, and with modern equipment.
In short, one guy just can't do it all. Be honest - even those of you who have run 'one-man shops' for years will admit to having a slew of other contractors that you work with.