The distinction we draw between original and textbook Latin doesn't mean textbook Latin is necessarily bad. Indeed, textbook Latin is a very necessary thing and, when good, it reflects fairly well most of the principles of real Latin.
But the distinction we draw is based mainly on two points:
First, texbook Latin often differs from original Latin in that it is simplified, except in quite advanced textbooks. The fact that it's simplified makes it feel rather unlike what you can read in actual Roman authors — just as textbook Russian would feel rather different from, say, Dostoevsky, right? But this is only natural. Students can't just jump right into complex language, so it's necessary to create more simple texts to teach them the basics. The teaching of all languages works like this.
Secondly, even the best modern Latinist will not be a native Latin speaker, therefore they can go wrong, even if it's only very occasionally and in very fine details.
Both of these facts make it so that textbook Latin can't be used as data to support theories about Latin (unless your claim is about texbook or modern Latin considered as a "dialect" in its own right).
Do you see what I mean?
This doesn't mean that we scorn textbook authors in general, but it just makes sense and I think Ørberg himself would have agreed with us. By the way, I think his Latin is very good.
