Whether to accept co-authorship for conference presentation after making only a small contribution? A collaborator sent me an abstract he's submitting to a conference, listing me as a co-author (middle position, implying not a major contribution).  It's for a conference presentation, not a manuscript, and it will not be published in e.g. a supplement or anything like that.

I've spent a fair bit of time with him discussing and planning the project, I'm listed as a co-PI on the grant, and I expect to provide important contributions later in the project, but at this point I haven't specifically contributed, in any hands-on way, to the exact work described in the abstract.  

I thanked him, made some minor suggestions about the abstract, and suggested that he didn't need to include me as I hadn't contributed enough yet.  He replied saying that he was happy to include me.

The work is fine and I have no concerns or reservations about it.  We're roughly at equal career stages and neither of us is new, so neither of us is looking for extra prestige etc. If we cared about fine balances, I'm probably helping him more than he's helping me in the project, but I'm happy to do it. I have made significant contributions to the overall experimental design and planning, and it's not ridiculous to be an author, but it is for a smaller contribution than I consider authorship-worthy.  

Should I continue to battle to be taken off the authorship?
My answer is based on experience in the field of applied mathematics.  I'm assuming (as stated in the question) that there is no written publication involved -- just a talk.
Don't worry about claiming unmerited credit
You have made some contribution.  Coauthorship on a conference presentation carries almost zero significance for purposes of evaluation.  It's not something you list on your CV, no matter how &quot;early career&quot; you are.  Usually, if you are not already well-known and the results are not earth-shattering, then nobody will even remember, five minutes after the talk, that you were a co-author.
When to insist on being removed
Obviously, coauthorship of the talk has some significance to you and to the presenter.  Since the presenter has already made it clear that he wishes to include you as coauthor, I would only insist on being taken off if:

The scientific work or the quality of the talk is so bad that you think it will reflect negatively on you; or
Your involvement is so minimal that you couldn't say anything interesting about the project (if asked by someone who attended the talk).

It doesn't sound like either of these conditions apply, so my advice is to let it go.