Pierre, first of all please wait till you can see a new function in MDL-20463. I've planned a serious improvments to it, but lacking time to implement them right now.
However, even reading the current patch you can see that $maxrepeats parameter could be set to FORMS_REPEAT_CONST, in which case user can't change anything and won't see appropriate controls. So it's already done (and this is not an option with current repeat_elements function). Please read the patch (at least comments section before the function) carefully.
About new input elements - first, only one of them is actually new (the button is there already), and they are on one line. I've just separated number from the button and give a user control over it.
I agree that this interface is much more suitable for match/multichoice question type, but having a two different interfaces for such simple occasion is probably more confusing (and harder to maintain) then help. So older function is likely to become deprecated and this conversion will have to be done. At least you would not bother youself with question "how many elements should be added by each time" - each user can set this according to his needs. Is this bad?
Actually you can see that this issue has minor priority while MDL-20463 and MDL-20464 - major. So you shouldn't bother about it at all before they are fixed. And then you can choose any arguments you want - I don't have strong opinions about using this in calculated question unit handling.
Please join the discussion in MDL-20463 once it started, and you'll get all desired functionality.
P.S. I don't see you on Jabber chat on moodle.org Have you a chat account? I would like to ask you some question about cloze question and how I can support my question type in it.
On a closer look of the patches in the initial issue i.e. MDL-17064, I think that this control of the repeat element added number is a good think for questions like match.
However for the calculated question, this will add another 2 input elements for an already over complex form.
Even at the actual (1.9) version the initial display has been limited to one answer .
The problem will be worst in the new calculated (HEAD) interface that offers an initial choice between editing the question as a regular calculated or as a multichoice calculated.
The same comments apply for the unit display in calculated and numerical questions given the new elements in the question interface related to the unit display handling and penalty.
These question types are not good candidates for numerous answers or units
I think we should wait and see if there are many complaints in the forum about the actual number of repeats element added number for these question types.