Imagine it: The MC has been obsessively worrying about telling best friend, love interest, mother, father, plumber some really big, life-changing, plot-centering news. She's sure that as soon as the news is out, this second-most-important-character-in-the-story is going to hate her forever, turn her in, whatever. But when MC and #2 are together, the author makes it perfectly clear that if all info comes out RIGHT NOW, the MC has nothing to worry about. #2 has gone through some pretty speech about how MC should trust him/her or otherwise proven his loyalty. But MC, for one reason or another, decides to say nothing and continues through most of the book lying to everyone around her.
Oh, we know why, dear readers. The ONLY reason an author uses this tactic is to create cheap drama. We know when we read it what's going to happen. The MC will push everyone away and go it alone for most of the book until the last chapter when she finally comes clean. And in the meantime, readers are screaming at the MC to wake up and just talk to someone! I call this The Soap Opera Syndrome.
Should this tactic be avoided by authors?
For the most part, yes. It's just plain annoying and everyone knows there would be absolutely no drama in the book if the MC had just spoken up in the first place. However, I have seen the tactic done well in a rare few instances. But it ONLY works if the author adequately places doubt in the heads of the readers. The same doubt that's in the MC's head. Usually this doesn't happen, and when it doesn't, the MC comes across as an idiot.
Examples of where it DID work:
Harry Potter and the Goblet of Fire. The tactic worked because Ron was being a little bugger himself and the reader believed beyond a shadow of a doubt that Harry confiding him would do nothing.
Speak. It worked because 1. the MC was surrounded by a bunch of twits who wouldn't have listened to her anyway and 2. because something traumatic happened to make the MC lose her willingness to speak up.
If you don't have a real, undeniable reason to keep your MC's lips sealed (other than to create drama), then don't do it. You'll only risk upsetting your readers and losing their trust in your abilities.