To me, the word "necessary" makes a pretty strong statement.
Having said that, I personally will read both indie and conventionally published books, although I will admit to a preference for conventional publication. What seals it for me is that the floor is usually higher with conventional publication.
I know that the writing in any commercially published book is going to display a certain minimum level of technical proficiency. Nouns and verbs will be used correctly, words will be spelled properly, and there will be very few errors involving homonyms.
In short it won't read like it was transcribed from an audio recording by a non-native speaker. Microsoft's speech recognition engine makes fewer errors than the author of the book in the OP.
I can't guarantee that for indie books. I'll still read some of them, but I'm enough of an editor at heart that I'm going to be very picky.