Its a very dangerous fire hazard because an excessive amount of lint built up inside of the dryer can be ignited by the heating element. Even though some lint will make its way past the filter and get into the exhaust, its not significantly enough lint compared to the amount of lint that is created with every laundry load on a regular basis.

Jack's answer is so true in that not regularly emptying the filter is just as dangerous as not having one. Folks, if you are missing your filter, purchase one immediately before using the dryer. The manufacturer can be contacted for replacement. If you still have your filter, PLEASE empty and scrub it regularly with only warm water. It doesn't need to be scrubbed after every load, but the more you let the lint build up in the filter, the harder your dryer has to work to extract the lint from your next load. Just pulling the lint off is not enough, especially if you use dryer sheets, which will create a fine film on the filter. That heating element will overheat, and the lint is very combustible. (This is why the girl/boy scouts use it to start campfires.) To clarify for Relsqui, Jack's answer for those who have filters is correct. My electrician has told me that some lint may get past the filter each time you remove the filter, as some of the lint fragments dislodge from the filter and remain in the channel. When the filter is replaced, it pushes those lint fragments down into the channel. The heating element burns them up, but it is in such small amounts that you don't notice. However, when large amounts of lint are sucked into the channel, the heating element burns that larger amount and then you have a crisis. It would be a shame to lose your home over a lint filter...

A couple of points concerning safety... Always use the filter designed to fit your dryer and clean it after every load. If the filter becomes damaged, replace it. You want to avoid lint having accumulate anywhere in the exhaust system, both inside and outside the dryer. Too much lint in the exhaust path will produce a partial blockage, causing the air inside the dryer to overheat, which may lead to a lint fire. The exhaust path between the dryer and the outside vent should be as short and have as few bends as possible, so as not to cause excessive back pressure. You should avoid running wire-reinforced plastic flexible hose between the dryer exhaust and the outside vent. A short piece between the dryer and a the main exhaust line is fine, but nowhere else. This type of hose tends to trap lint and produce more back pressure than smooth metal tubing. Metal flexible tubing is much superior to the plastic type.

f the filter is missing the lint is going right into the blower and out the back into the vent pipe. When that pipe fills up and stops the flow of air the machine will take twice as long to dry the clothes, Eventually the heat will buid and blow the limit. If the limit don't blow the fuse will. Unless these have been bypassed and that is when the fire occurs. Dryers have a lot of redundancy built into them for safety. You would be a fool to use the dryer without a filter. No matter how safe a dryer is , it is no safer than it's owner.