Exposed ceiling joists can certainly impart a quaint olde worlde feel to a building, but since they are also the floor joists for the rooms above, the shrinkage gaps between the floorboards will allow heat to pass upwards, dust to fall downwards, and noise and smells to pass in both directions. No wonder our forebears preferred to cover them up.
Timber studwork in partition walls is also better covered over with lath- and-plaster. The only time it would have been left on display would have been in very poor housing, where the stud walls were plastered only on one side; the occupants of these were usually so embarrassed at the unfinished look that they covered the exposed studwork themselves with flowery wallpaper. Many examples remain, but you won't find them featured in the conservation journals.
In real construction terms, a beam is a horizontal member spanning across two columns in a framed building, and as such, it will usually be hidden within a wall, so on the rare occasions when an estate agent does correctly identify an exposed beam, it will usually be a secondary beam, supported by either columns or main beams, and designed to reduce the span of the floor joists above. In many cases it will have been put in to pick up the joists during alterations, taking the place of an earlier partition wall.
Timber secondary beams in old pubs often sag in the middle. This is not a sign of imminent collapse, but because of a phenomenon known as "creep", which occurs when fibrous materials such as wood gradually distort under load. This is what gives old timber-framed buildings much of their character. If you see a beam in an old pub which is not sagging, then it is probably not a timber beam at all, but an RSJ (Rolled Steel Joist) cunningly disguised with timber or PVC boards.
A friend of mine has recently made an involuntary close study of exposed secondary beams in a Suffolk country pub. Intoxicated by the atmosphere of the late-night revelry, and driven by the foot-tapping rhythms emanating from the jukebox, he started to dance. But, as all country dwellers know, people under beams shouldn't pogo. Lucky for him that the "beam" was a fibreglass fake, or the blood loss could have been even greater.