The commonly installed neoprene rubber stack flashings may be acceptable for a few years when used to flash the plumbing vents where they exit the roof. But these flashings are prone to problems, including deterioration as a result of the harsh environment up on the roof -- UV rays being primary, but also wind. This is a problem I run into frequently at inspections in Whatcom County and Bellingham.
If they are used with a roofing surface that has a fairly short design life, then they might last about as long as the roof or develop problems simultaneously with the roof. But, when they are installed at a longer lived roof, such as architectural composition, these flashings are likely to develop serious problems prior to the roof developing similar issues.
In the photo above, the rubber has deteriorated. This will, often, lead to holes around the pipe and, on occasion, I have found the holes at stacks to be sources of significant leakage into the house below. Any homeowner should, periodically, have the roof checked-out by a professional. And that professional, should check all flashings and cite potential problems that can lead to water damage.