How useful is it to have English as a B language instead of a C language for an Italian mothertongue?

I'm not in Italy, my IT is only a C and my EN a B, but I don't think that the answer to your question depends on level-matching or is language/market specific, for that matter, so here goes:

• the combination with which you'll start your professional life will be the one with which you'll come out of your studies, ie pass your final exams, not necessarily the one you'll postulate as you go in :-);
• graduating with a C means just that... any language can be upgraded depending on one's level to begin with, if one works at it from early enough and does so hard enough... and a C to B upgrade is in theory a reasonable albeit time/effort consuming ambition;
• you most certainly can improve your professional EN after your graduation (but it must be strong enough to graduate with, albeit as a C), you actually should and most likely will... and that goes for all of your languages/cultures, by the way :-);
• an EN:B does indeed open the door to more work opportunities, "more" as in more than w/o such a B or with some other B: I think it's fair to say that "national language:A+EN:B" is THE winning bi-active combination in most national non-agreement markets: it obviously does not mean that w/o it you will be barred from a successful career, only that with it (all other requirements being met) it'll be more likely... and professional careers are not limited to (bi-active) national non-agreement markets either, even less so to any one such market.

This being said - and hoping to be forgiven by EN:A colleagues for hazarding an opinion in this regard :-) - do remember that EN is both a very "easy" language to pick up for basic communication purposes AND a very difficult one to acquire professional proficiency in, all the more so if active... and then there are the many EN-speaking cultures underlying the language; moreover, often times students obsess over their "foreign" languages and loose sight of the fact that one's A language is one's main trump card... and a lifetime endeavour in its own right. Good luck! :-)