There are some very compelling reasons to clean your dryer vents:
These problems are created because the lint filter on any dryer only catches part of the lint created in the drying process. (Usually about 75%) Much of the lint that gets past the filter on the dryer accumulates along the walls of the vent. This reduces the outward air flow which in turn alters the performance of your dryer. Given enough time the restriction can become a blockage.
The restricted air flow increases operating cost, causes more wear on the dryer, and extends dry time.
Heating elements and fans are overworked to the point that efficiency is lost. If the restriction is severe enough it can come to the point that the dryer just does not work. It runs, but clothes do not get dry or do so very slowly.
We estimate that most dryers that have been in use for six months to a year are effected. Our experience has shown enough lint accumulation to warrant cleaning even sooner in some cases. With heavy use it does happen quicker and with time the effect becomes more severe.
Lint accumulation in a dryer vent can not be prevented. To have an efficiently running dryer the vent must be periodically cleaned.
After some study we concluded that there is a real fire danger. At first we thought this was just a scare tactic drummed up by marketers. Our equipment supplier checked with his fire department and was told there are 15,000 fires a year started in dryer vents. Farmers Insurance had an ad that stated the same thing.
Whatever the numbers it makes sense to be cautious. The lint that accumulates in the dryer vent is very flammable. If you ever need "tinder" to start a fire with flint and steel you can find it in your dryer vent. Just a spark or sufficient heat of any kind can get a fire going quickly, especially with some air movement to fan it.
The benefits are really there . . . It is easy to see the value of keeping dryer vents clear.
We recommend cleaning dryer vents at least once a year. This is about the same as cleaning your carpets with normal use. As with carpets, more often is reasonable with larger families and heavy use. Long or twisting vents can also play a part.