I was shocked.
In the limited instances where wood-meets-earth or the worst precipitation (window sills, balustrades and stair treads), PVC composites may appear to be an attractive alternative. But its use should be considered with caution; new composite risers, fascia plates and sills will not show signs of degradation, but they will conceal what's happening to wooden structural members behind them. So, when it fails, be prepared for wholesale failure of the system. ("Gee, the stairs looked great ... who knew the stringers were rotted?")
Like many issues in restoration work, this one can be attributed to a lack of informed sources. The building owner typically knows only as much as the contractor has told them. PVC composites are widely available and spend a tremendous amount of money "educating" would-be consumers through marketing. Consider the Secretary of the Interior's position on the subject:
"If repair by stabilization, consolidation, and conservation proves inadequate, the next level of intervention involves the limited replacement in kind of extensively deteriorated or missing parts of features when there are surviving prototypes (for example, brackets, dentils, steps, plaster, or portions of slate or tile roofing). The replacement material needs to match the old both physically and visually, i.e., wood with wood, etc. Thus, with the exception of hidden structural reinforcement and new mechanical system components, substitute materials are not appropriate in the treatment Preservation." (Underlining added for emphasis)
The use of PVC composites is an attempt to cut corners and remove maintenance and upkeep from the equation. Consider the use of Spanish cedar, mahogany, oak, and other hardwoods in these limited applications. You may be surprised how close the price is to the composite materials.
 Standards for Preservation and Guidelines for Preserving Historic Buildings (http://www.cr.nps.gov/hps/tps/standguide/preserve/preserve_approach.htm)