Far from the most important point, but easily my favorite one-liner:
Would you pay $100 an hour for an untrained accountant? Because if your consulting rate is $100 an hour and you do your own accounting, that's exactly what's happening.
That ignores the tax implications. If you earn $100 as a freelancer, you might get $60 after tax, which means you'd have to work one hour and forty minutes to pay your accountant $100 (edit: no. As aaronblohowiak notes, you would have to work the same hour, because accountants are tax-deductible). And there are frictional costs -- I can hire myself as an accountant for free, but I have to at least get referrals, make phone calls, sign contracts, and write checks for my accountant.
So it's a little more complicated than that. Maybe "Would you pay an untrained accountant $60 (edit: $100) per hour, with zero transaction cost, no agent-principal conflicts, and instant availability?" Maybe not a good deal, but still a better deal.
For me, there are two qualities I look for when hiring an outside service provider like an accountant which go beyond the $/hour perspective.
1) I assume my time is essentially priceless. If I've ever had complex finances, for example, I hire an accountant because it's not worth me trying to decode some obscure tax law (which commonly changes year to year btw) for several hours just for the satisfaction of knowing I've DIY'ed the situation and maybe saved a couple of $ in the long run, while quite possibly also having had made mistakes.
2) Speaking of mistakes, the second reason is that often times hiring someone else comes with some kind of guarantee or warranty. Hire an accountant and if your taxes aren't right you're liable to make up the error, but you're not going to jail for tax fraud. Hire an electrician and (usually) they'll stand by their work and come fix errors within a given amount of time.
Not saying I disagree with your statement, just offering a perspective on why $/hour doesn't always evaluate so clearly as a meaningful metric.
"If you earn $100 as a freelancer, you might get $60 after tax, which means you'd have to work one hour and forty minutes to pay your accountant $100." - um, isn't the accountant a business expense and therefore a writeoff?
I do my own because I wrote software to handle it. Whenever I discover a new piece of tax code/legislation or reporting requirements, I encode it in software so I don't have to do it manually ever again.
Would you pay $100 an hour for an untrained accountant? Because if your consulting rate is $100 an hour and you do your own accounting, that's exactly what's happening.