Big difference? Depends on what's in it and who you ask. I've never met an injected butt I liked at all. Imo, injection is wholly unnecessary in terms of tenderness, as you point out, Davidd; in terms of flavor, what injecting butts does is add them at the beginning where they cook (or not) and bind with each other (or not) during the smoking process. It is far better, in my opinion, to add to or adjust after cooking, resting and pulling as one has far greater control over the end result.
That said, there are quite a few competitors who use it, but competitors are trying to make their submission resonate with judges' palates in a bite or two--in a field of other competitor's submissions--and so make choices in terms of procedures and ingredients that they feel will facilitate this.
In my opinion it's overkill, competition or not, quite frankly.