It is at least better than writing a software that does not work for some configurations. That is what the operating system is good for.
It's not when your target is AGA+68020/30 and possibly A1200+trapdoor fastmem. Why do we have to take higher end systems in account, and make sacrifices on the low end, while the software could work perfectly fine on low end?
This is Amiga we're talking about, not some high end computer where making use of the OS for everything might make sense. For low end (68020/30) using the OS is fine. Well behaved software is nice, after all, and killing the OS (except for WHDLoad and demos) isn't a good thing, same for not using the OS's screen open functions. But blitting has to be fast on low end machines, or the software is going to run like crap.
Why does everything have to be adapted for high end machines and custom expansion boards? Want to run Amiga software? Use an Amiga.
The main advantage of using the OS functions for blitting is that it works asynchronously so that the CPU can multitask while the Blitter is still plotting graphics.
With the blitter, yes, but the blitter is too slow.
Thorham's approach is the old-fashioned way of using the CPU to copy pixels. It works best when you have sufficient cache memory and CPU time to do it that way.
It's also the only way to get good performance on low end machines. It's old fashioned because the hardware is old, and many Amiga users use this hardware.