FIRE HAZARD
Lint build-up can cause reduced airflow, which in turn causes overheating. When this happens the high temperature limit safety switch cycles on and off.
Most switches weren't designed to cycle on and off continuously which can cause them to fail.
The Consumer Products Safety Commission estimates that there are an estimated annual 15,500 fires, 10 deaths and 310 injuries associated with clothes dryers. This also produces over $84,000,000 in property damage.
Accumulation of lint and reduced exhaust airflow provide conditions ripe for a fire. Lint is highly combustible. Lint is also one of the main ingredients in a recipe for home-made fire starters.