The poly tarp material itself is generally the least expensive out of the variants of material available, so affordability does make it popular choice for those who are wanting to save a little extra money. Polyethylene is also waterproof and typically used for tent liners and covers as a little extra protection in the case your tent springs a leak in foul weather.
The light duty poly tarps do provide adequate short term protection, particularly when temperatures are on the cooler side. Prolonged exposure to wind and high temperatures may decrease its lifespan. If its intended use is null of the elements, the shelf life of the product may increase quite a bit.
Polyethylene tarps may be a perfect temporary solution for short term usage such has temporary covering for exposed roofing or outdoor storage coverings. If something is needed more long term, it may be wise to consider a more durable product such as canvas or vinyl.
One of the biggest pros to poly tarps use is that it is cheap and cost effective. Another pro is that pretty much anyone can use it for a wide variety or purposes and it is readily available. The biggest con is that it is not as durable in the elements for longer term use.
When looking at the pros and cons of poly tarps, it is very important to consider what you intend on using it for. While you may choose it because it is inexpensive, it may not serve your long term needs.