Fire Code dictates that you must have an access point (panel) every 12' of liner duct-work. The reason for this is that you can't clean what you get into to. For areas where there aren't any access points you run the risk of grease buildup
This build up of "fuel" will increase the chances of a grease fire taking place. If a fire takes place in your duct-work, then it will ultimately start a fire on your roof. There are 6000 to 7000 grease fires annually across the USA due to grease accumulation in the duct-work.