Here's my suggestion, it is a bit long.
Your Echo is something that I understand from your descriptions, car that you want to last... especially the transmission. That is why you would consider the fluid filter right?
Well the inside has a filter mesh screen (debris) and magnets to hold ferrous(iron/steel alloys). I personally change the ATF and screen filter, and clean off the magnets on the second oil change, (First Oil change at 1000 km, next is 6000kms, conventional). This way all the debris from burninshing the moving parts are removed.
The bypass filter can filter out the finer particles and non magnetic bits like worn friction material in the clutch bands etc. but the transmission is not built with such close tolerances that these would affect its life dramatically.
You will have clean fluid.
The statement from those that said 'add a cooler' is preserving the transmission in another aspect. Keeping the fluid 'good' so to speak. The fluid's chemical composition can be better maintained for longer life if the temperature is kept lower. The Additives that maintain the fluid will do it's part in keeping the transmission working. Clean fluid with worn out (burnt) additives is also not good.
Summary: Transmission needs AT fluid to preform the 'lubrication' 'flushing/ cleaning' 'sealing' 'cooling' functions.
Bypass filter help with flushing and cleaning.
Cooler compliments the system and adds to fluid life.
Both are advantageous, but remember fluid pumping can decrease fule economy because you need the engine power to move the fluids.
The costs of a bypass filter may not out weigh the 3 year/ 60,000km change if it is around $80 CDN each time.
The cooler would be worth it though depending on oyur conditions.
[ May 14, 2004, 10:10 AM: Message edited by: Hajoca ]