I primarily shoot Glocks in 9mm, but own a 23 too. I had been casually in the market for an RTF Glock when they'd come out, then it was announced that sales would be curtailed to LEO only. I came across a Glock 23 RTF on sale, and scooped it up figuring I could use the frame, change over to a Glock 19 slide and ejector, and then have my Glock 19 RTF. In the end, I didn't do this since I found that the Rough Textured Frame was a little too roughly textured, digging into clothing that it came into contact with, and punishing my hand after longer range sessions.
While I never made the conversion, I've also kept the 23, bought a couple extra magazines and some ammunition for it, and keep it to this day. It doesn't get shot much (more expensive, less enjoyable, and it doesn't do anything the 9mm guns won't), but it's also been 100% reliable.
Reasons to keep it: It could serve the role of a compact handgun if my Glock 19 goes in the shop. While .40 isn't my favorite caliber, I don't dislike it. Being the only RTF Glock I own, it's unique from a collection point of view. If 9mm ever goes dry (election-style), but a local store still has .40 on the shelf, I can still go shooting.
As far as similar magazines go, I keep mine separated. Most of the G34 magazines reside in the competition bag, G19 magazines in the same bag but their own compartment, and .40 magazines that don't get pulled out much get their own spot in the closet. Whenever I pick up a gun and verify that it's status, I'm also verifying it's magazine, and double checking any spares I'm bringing along. CCW or HD guns always travel with at least 1 spare, which gets double checked every time I pick it up.