Yes, press conferences are important. They show that the candidate is honest, forthright and transparent. Having more press conferences would increase voter confidence in this candidate. She can always take a question and provide an answer that puts her in the best light. People want an accessible president, and press conferences are key to accessibility.
The Clinton campaign is a well-oiled machine, presenting a carefully prepared image to the public. Having more press conferences with spontaneous questions by the press would help Hillary to appear more approachable. There are often complaints that she is not as likable as her gregarious husband; being more available to the public would help people to know her and like her a little bit better.
Further, press conferences are expensive to produce, and not just because of the air time. Rather than having something to hide, it may just be that Clinton doesn't have very much to say. Also, given the current legal scandals she's involved in, how many times does she have to say publicly that her lawyers have told her not to talk about her legal problems. Tough, in an election year, sure, but she has the right to competent legal defense as well.
No, the Clinton campaign should not necessarily hold more press conferences. Press conferences are meant for announcing important events or changes. They're not intended for the sole purpose of promoting an individual. There are other avenues for that like social media (Trump campaign excels at this) or public appearances and speeches.