Even though the lint trap catches some lint in the trap, over time dust and debris still build up in the vent and get trapped there. For example, facial tissues and other forgotten items run through the dryer cycle and create quite a mess. Many vents also have a screen cap to prevent rodents, insects, and/or birds from accessing your vent, but they also make it harder for lint to blow out of the trap.
Yes, dryer lint is highly flammable. When lint and other debris build up in a dryer, they reduce airflow and back up exhaust air into a hot dryer — a bad combination.
Unfortunately, no. Gas dryer vents also release carbon monoxide, but if the vent is clogged, it may cause a dangerous buildup of carbon monoxide, putting you and your family at risk.