STI = no rev hang
The rev hang is programmed in for 2 basic reasons.....
Emissions related
When the throttle is released and the clutch is dipped, an ECU with rev hang is programmed to leave the throttle slightly more open than usual.
This strategy is used because it was found that suddenly closing the throttle produced a sudden spike in gas pressure within the crankcase which emphasised the rate of oil vaporisation, thus adding to emissions from the engine. Also, when the throttle is suddenly shut, the fuel mixture suddenly becomes lean which creates NOx (oxides of Nitrogen) which also contributes to emissions.
Only being done in gears 1 and 2 is because gear changes from 1 to 2 and 2 to 3 are the most common and frequent changes. Higher gears see less use for the average motorist, so less changes means they can exclude the rev hang from those gears' programming without worry about an emissions scandal.
New to stick drivers
Drivers who are new to driving stick have some difficulty shifting. In some instances, the rev hang can help provide a budding stick driver to shift with less issues. Unfortunately, once you get used to driving stick, and start slam shifting as fast as you can, the rev hang is an annoyance.