Lurking is a profoundly defensive strategy. In exchange for not providing any information to the Town, lurkers render themselves very difficult to eliminate due to the lack of information needed to build a convincing reason to eliminate them (beyond the obvious excuse "they're lurking"). These are both negative traits as far as the Town is concerned; in addition to being unhelpful for finding others' alignments, they're unhelpful for displaying their own. Thus, lurking is a preferable strategy for scum if they are allowed to get away with it, as they provide no help to the Town and can simply wait as the active players self-destruct on each other.
Thus, lurkers are considered infuriating to deal with from the Town's perspective, and players who lurk regardless of alignment are usually considered bad players (if not bad people in general). However, some players manage to justify lurking even as Town. Power roles such as Doctor or Cop may deliberately avoid posting so as to avoid the scum's notice, thereby avoiding the nightly kills until it is time for them to claim. Alternatively, some players believe that it is best to sit on the sidelines during Day 1 and pick up the pieces on subsequent Days after there is information to go on.
Pressuring lurkers with votes and wagons generally does not avail much, as the lurker knows the votes are only there for pressure. The best way to deal with lurkers is via process of elimination - if you are able to roughly gauge how many scum there are in the active players, you can guess how many scum are in the set of lurkers. This tends to work better if the vast majority of the active posters are Town-aligned, though. In games where there are several scum in the active players and/or the lurkers are mostly Town-aligned, consider yourself hosed.
As scum, lurkers are best eliminated when the Town does not have any strong alternatives. Since you know who the scum are, you can direct the elimination away from your lurking partners.
Lurkers are harmful to the town, as multiple town lurkers allow scum to also lurk and not be eliminated. This results in scum being unable to be read, so it is more likely for town to be eliminated.
See also Active Lurking.