Depends on what you are doing I suppose.
If you are just working on your own stuff you probably don't need them, but they would still be handy.
If you are working on vehicles for a living I would say you need them.
They are fast and handy to grab because they will generally work whether or not you need a swivel, but a straight socket won't work if you need a swivel.
Also saves the time of piecing together a swivel and a socket when you need one, and usually is smaller making it easier to get into tight spots. Especially true with the smaller 1/4" sockets.
The three racks of sockets I use the most are the 3/8" shallow impact swivels, 3/8" deep impact swivels, and the 1/4" shallow impact swivels. (or universal if you prefer) I keep the shallow 3/8" on a rack I can grab and take with me
I also have a set of the medium impact swivels from gray pneumatic I don't use a lot and keep in their case in a drawer, but are very handy when you need them. I know another mechanic who uses his mediums the most, not sure which way is better.
I do mostly engine work so I don't really need the 1/2 impact swivels, but do wish I had them from time to time when I do suspension/trans work.
I really wish I could find a set of 1/4" deep impact swivels.
The 3/8" deep and 1/4" sets are sunex and are holding up well with daily use for several years.
Only issue is I really wish the 3/8" deep set had an 8mm, but they only seem to offer that in a 12pt.
I prefer 6pt for impact sockets, and all of mine are 6pt, except for the special ones - like the driveshaft universal sockets.
The shallow 3/8" set is genius, has an 8mm. It was cheap and is really old and all the sockets are still functioning fine. Will say I like the sunex feel better.
Only real annoyance I have had with the universal impacts is when I end up trying to use a swivel head (especially roto) ratchet with them. Good way to pinch yourself I've discovered.
Edit: I bought a set of the harborfreight universal impacts because they were really cheap - I later discovered their was no 15mm. A socket I need a lot, so it was basically a waste of money.