Yeah I agree, I don't even know where my PSTV is anymore.If you already have a Vita I'm not sure; the appeal of the PSTV is having cheap consolized access to the Vita library.Playing on the big TV screen with a regular ps3 or ps4 controller. I got mine for $25 back in the day when Sony was having a sale and I didn't own a Vita yet and that was it's major value for me.
I used it primarily to play Persona 4 Golden and Freedom Wars. It plays most Vita games at 720p.
I'd say it's worth it if you really wanna play Vita games on the big screen from your couch, but otherwise pass since you already have a Vita. I like mine, but I don't use it much these days. I'd also recommend checking to see if the games you wanna play on it support it; there's a couple Vita games that don't work on it, like Gravity Rush. (You can hack it to make those games work tho)
I don't have any use case for it.