Tim blathers. But not about everything.
Sure, you catch him in the midst of a thought and he starts telling Gibbs all about the finicky plot bits in the Star Trek reboot (and how much Uhura kicked ass, even though her translation of the overheard transmission totally added another verb tense into the Klingon morphology and--) or if Abby surprises him while he's catching up on his blog feed and he spits out way too many statistics on the likelihood of one's technology rising up and killing you, not even getting into the whole SkyNet possibilities, because that would lead to an inability to sleep and the need for a fuzzy blanket because Jesus, how scary is that.
He has a bad tendency to blather on stakeouts. Or whenever Gibbs is in earshot. Especially when he doesn't know that Gibbs is in earshot.
But it's not universal. It may not sound it (he knows it doesn't look it), but even when he's talking, rambling, spouting theories and stories, his brain can offer up thoughts that his mouth won't ever share. When Gibbs tells him to wake up Ziva and Tony, get them out of bed, and his brain is thinking bed, Tony will be half-dozing and sleep-warm on Tim's bed, pulling at him with a quirked smile that means he's ready to hear Tim wind down, sputter out complaints about Gibbs' hectic schedule and never ending energy until Tim could stop, lie still, rest, slide his hands to warm against Tony's stomach, press a kiss to the back of his neck... his mouth instead spills out stammered jokes about Tony and Ziva and beds, a handful of barely coherent sentences until Gibbs releases him with a look and flicker of flashlight glare.
Gibbs always trained them to be specific when they lie (rule number 7), and Tim has no problem taking advantage of all the (trivial, inconsequential, usefully decoy) details at his fingertips, especially if it lets him keep one thing to himself.