I have a Glock 22 with a Wolf 9mm barrel and 357sig barrel it shots both well, mater of fact I've never shot it with the factory 40cal barrel. I also have a 17. The only reason I bought the 22 and other barrels was to put it my SHTF bag. If you just want to shot 9mm I'd go with the 17.
If you're already heavily invested in .40 and/or already have a G22, and only want the 9mm to plink around with - then the conversion is a much easier and less expensive way to go.
You can also convert that G22 to .357 Sig if desired. That's a real honey-pie of a round out of the full sized Glock.
It's practical, sure, and a decent investment - but if you are open to the idea, it's not a bad investment to buy the 17 either. Just depends on how much money you want to spend..
I have a G35 with a wolff 9mm conversion barrel and it works perfectly. I think its a good thing to have as part of your Glock accessories. It gives you just one more option.
It just doesn't make sense that I can get a 22 gen4 for $375 used very lightly put the conversion barrel on for $125 but a 17 is $500 and you have two very popular calipers