All in the title and this doc
http://www.idpf.org/epub/20/spec/OPF...m#Section2.2.6
additional doc:
http://www.loc.gov/marc/relators/
To make short: <contributor> looks like a <identifier>, but has a standardized set of "type".
It is normally in this place, that certain thing as must find. Ex. the translator , the Cover designer, the illustrator and many other thing really useful.
I find that such forgetfulness is very annoying... but this can understand because we are talking about +200 reference, has very specific use.
It's never too late to add it, and it would be really useful, so:
Add a new input box like as <identifier> is not recomanded, because the data entered are most particular.
No, it takes something bigger than a small rectangle for that.
For me, the best solution would be to add second tab in the metadata edition, like for
Custom Colums.
To avoid overloading the interface (and user) with the +200
Contributor Type, only
Contributor Type is defined with a value will be shown.
The value in the "text box" will be treated as for Authors (text separated by "&")
[a one mark <contributor> for each name]
At the bottom of the tab, a button "Add a contributor type".
When we click on it, a small window allows us to choose a
Contributor Type to edited among those not displayed (in fact, it forces the display of a empty
Contributor Type. If you change of book without having entered any value, the
Contributor Type as gone)
And for the columns, how to do it? because adding +200 is a littel lot.
A practical solution would be to do like the ISBN example.
If you want to display the
Contributor Type "Cover designer", you create a
Custom Colums (Column built from other columns) with this model {contributor:select(cov)}
But unlike the identifiers, we will certainly need the commas in the
Contributor Type, so it can't be used to separate them. Personally, I have no idea how to display and edit all of the
Contributor Type in one line (and I don't think that's a good idea)
Finally, an option (probably in Tweaks) to force the display a liste of
Contributor Type, even if empty.
Ex
contributor_type_forced_display = ['trl', 'cov'] will force the
Contributor Type Translator and the
Cover designer, even if empty.
Probably a big addition for few users, but
it's a official specification for ePub 
I'm not saying:
you absolutely have to add it because, here's the thing, no reason
No, I think we should
seriously consider and think about how adding the support from <contributor> mark.
This would be one more step towards better support of ePub format, and wider and universal growth of the information/metadata contained within.
All of this is a idea and just suggeste. I would be very happy to see a support of <contributor> in Calibre, in Calibre itself or whas a plugin.
If some person wants to try this, I thank them for it.
Good continuation