I can sit with my butt touching someone else's on the subway for an hour, but would hardly call that "spending time together", or presume to know anything about them other than their smell.
Also I think many extroverts misunderstand introverts in this respect. My mom (introvert) and me (introvert) would often just read in the same room, and that was enough for us: I assure you, it felt very different than sitting with my butt against a complete stranger on the subway. My younger kids (introvert and ambivert) insist on digging their toes under my back while we sleep to make sure I haven't gone anywhere; they are not even conscious, but the proximity is important to them. This particular kid is an extrovert and has made about a dozen bids for my attention in the time between my previous comment and now (most of which have been reciprocated), but that is again the huge difference between sitting next to a family member and sitting next to a stranger on the subway. With family, that bond of trust is already there, and so you feel okay interrupting the silence to ask what your next move should be in Magic Sort, or why you can't download Township, or how do you turn the phone off silent mode, or showing me how he just beat his personal best score on 1 To 100, while you would never ask those of a stranger on the subway.
But I agree, for some people just reading together in the same room is enough.