I've read a bunch of people use this round in IPSC.
True. But that really has very little to do with carry applications. Power factor in IPSC/USPSA is simply a momentum calculation. And that's a game that, in many classes (especially Open) rewards high ammo capacity. It's easy to get a high power factor for a big bullet, like a .45 200 gr SWC, but capacity will be limited (fat bullets take up more room). In order to get a lightweight bullet going fast enough to "make major," .38 Super was about the only answer for a long time (barring bottle-necked rounds). Since they dropped the floor for major power factor a little, some people load 9mm up WAY past SAAMI specs for "9mm major" loads, but those are scary. I saw a 9mm major case rupture just last night! If you don't want to do that, then .38 Super is the easiest (and probably best) way to make major without sacrificing capacity.*
The other thing is that most IPSC/USPSA limited- and open-class guns are based on 1911s. That's not as universally true as it used to be, but it's still pretty true. 1911s generally work more reliably with cartridges that have a length similar to a .45ACP. 38 super is about that length, so it's a little easier to make a 1911 run .38 super reliably than .40S&W or 9x19.
None of that means that .38 Super isn't a great carry round... just that the things that have made it popular/optimal for some IPSC/USPSA guns don't necessarily have much bearing on, say, terminal ballistics.
*Note: USPSA has some minimum bullet diameter requirements in most classes that make .38 super ineligible for major scoring, but it works for race guns. And actual IPSC, IIUC, allows .355-size projectiles for major in all classes.