Part of the problem is having the kernel involved in starting shell scripts; convenient in some ways, but V6 etc worked fine without that 'feature'.
That's a tough call. I have mixed opinions about it as a feature. I kind of like the user/kernel interface really, really thin and unadorned.
Adding, it certainly made allowing multiple interpreters to be supported much easier; but as you say it opens a new can of worms. Given the later proliferation of what bwk or Jon Bently once referred too as 'little languages' (awk, perl, tcl, and sigh eventually python), it was probably a good feature.
But as you said, v6 worked fairly well without it.
Clem