And an ecm can drop your running temps by 30*. Stock fan comes on at 208* coolant temp. Dropping 28* to cycle the fan at 180* makes a huge difference. On power, legs and longlivity.
You can never replace the throttle response and acceleration of proper tuning. A piggyback device actually trades throttle response as it retards injector timing. And as the stock ecm changes the base pulse width (for temp and altitude) the tuning from the piggyback remains constant. It always needs retuned. An ecm, is just like the stock one and compensates for everything.
As wake said, higher rev limit in low range gives a higher speed and a much more usable low range. Mudders, snow and tight forest trails make low a nice fun gear. Speed and rev limit in high is only for high speed riders. XMR bikes are speed limited at 65 mph. Guys hate it. Its great in the mud pits but sucks when a little honda passes you on the way there.
Most builds take peak power up near or over the rev limiter. So when properly clutched, you are out of revs on a stock ecm. (09+ cams (intake/filter)and a good exhaust make peak power near 7800 and only drop 2hp at 8500rpm before starting to drop. Canam even upped the limiter to 8250 so riders wouldn't bounce off an 8000 rpm limiter when properly clutching an uncorked bike. Alot of better engine builds will take peak power to 8500 or so. If you dont clutch and run there, you are loosing alot of money you spent.
And as mentioned, the 2009+ bikes can have the low end snap/accleration enjoyed by the older bikes. All year bikes can enjoy more snap. Or even have the option of detuning by a toggle switch when the wife, kids or real technical situation comes into play.
Timing is very dependant on what fuel the rider wants to use. Stock detuned timing is set for 87 octane use and the knowledge that guys will have old jerry cans around or a bike that sits all winter and the use the old gas come spring. And its not near even 87 octane anymore.
I run 2 timing tables by toggle switch. One for race gas and maximum power and at the flick of a switch i can pull up to the pump and put 87 octane in it.
Getting rid of the reverse override is a nice addition.
Some people like to tune up the cruising areas better than stock (and stock wot is too rich as well), just so they can enjoy a longer range on a tank of fuel.
Some people like to be able to monitor every sensor, live on a laptop. Either for diagnosis or just because they can.
By using an ecm and datalogging, you can watch your rpm, time and see complete runs and pinpoint your clutching to a tee. See exactly what is happening. overrev, belt slipping, shiftout change - everything on a graph
Auto tune like nothing out there. If any mods are done at all - the engine needs tuned. What else are you going to use? Download a fuel map? guess at it and push buttons? buy expensive equipment or go to a dyno (if there is one near). Use a pc5, hope it works and then do one cylinder at a time? (both cylinders effect tuning of the over. injectors are in the same intake)
Overall - complete control and get every ounce of power, performance or fuel milage from your machine. Stock to modded.