Hmmm, well, I do see value in playing original tunes note-for-note if that's the way the songwriter wants it. I try to respect the writer and get input if I start to diverge or play anything odd. My band has a dozen or so good tunes and the guitarist is the writer. In spite of the fact that he teaches guitar, he's not much on theory so I tend to throw some theoretical ideas out for bass lines and chord changes when things are still in flux. I am always cognizant that the song is his brainchild and try to stay true to his vision.
That said, every situation is different. Am I coming in as a full and equal member of the group, or added on as a bass player to an established group where equality can come down the road? Does my view of the situation match theirs? If I want to stamp my style onto the tunes and they want at least an approximation of a note-for-note rendition, then maybe this is the wrong situation. Who knows...
As far as the used me to cover three gigs comment, I don't know. I suppose they could have used you like that because you had a pulse and could play while they continued to look for someone they liked better. If that's what happened, it's wrong on so many levels. When you first hooked up with them, I thought I remember you posting that it was a decent bunch of guys and a happy environment. If they wanted to bring you in just for a while to see if it would work, it wouldn't have been so bad if they were up front about it. Sure doesn't seem like they were, though.
Additionally, one can never know if the reason they gave you for splitting is the actual reason. Especially with them being so closed-mouthed about it now, you have to wonder if there is maybe another reason they aren't comfortable talking about.
In any case, it doesn't matter now except as a learning experience to help you have a better outcome from your next band situation. Good luck with it!
-bob