Not necessarily true at all Zac in my opinion.

A good TC is indispensible, it increases your focal length significantly without reducing the image size at all - it then allows you to crop more and you can apply a littel extra usm if required.

I have a Kenko 1.4 Pro G and yes, I think the IQ is very good, as good as the Nikon 1.4 in my opinion. Not built as well though. AF is a tad slower than without one, but not too bad. Works fine all all Nikon AF and AF-S lenses

The Nikon 1.7 that I have is good also, stopped down a little, and the AF is a little slower again (especially acquiring, but tracking is fine).

I have only ever tried a older Kenko MC7 2x and it was horrible