When I edit someone's code block with a language identifier:
Is that change reflected in the Suggested edits queue in the Inline or side-by-side modes?
Sure, it will be reflected in the side-by-side Markdown mode; but no one will have the time to check an edit in the Markdown content. Especially if the change is not shown in the Inline mode or side-by-side mode.
If someone doesn't see the change on the default side-by-side mode and decides not to view the Markdown, the edit has a chance to be rejected.
Why bother: Language identifiers are a good feature of the Stack Exchange network, and it helps to understand complex code and for beginners - all in all if the OP likes it; if the OP doesn't like it, then the OP can reject it.