Switching DNS is not harder than switching registrars, and keeping them in the same place is reasonable if that provider offers good service for both or you want to avoid the additional complexity of an additional vendor.
I thought your original comment made it sound like there was some obvious reason for keeping DNS and registrars separate.
Switching DNS is harder than switching registrars, because there's more records, and you have to transfer them, and you ideally want the old service to send NS records pointing to the new service, and the old service should be cancelled only after sufficient DNS traffic has moved to the new service. For a seamless transition, you need a period of time with both services active, but some registrars with included nameservice will cancel your nameservice immediately when a domain is transfered to a new registrar; and you likely can't start service at the new registrar in advance either. Recursive resolvers do cache and use glue records, and 2 day TTLs are common at TLDs.
On the other hand, a registrar transfer is usually simple and quick and has no user visible changes. Unlock the domain, get a transfer code, do any confirmation stuff, make sure the glue records didn't change, you're done.
All of those arguments are arguments to use a good DNS provider, not specifically to not put the domain and the DNS on the same provider. I've seen DNS providers remove the namespace the instant they see the domain pointing somewhere else too.
I recommend that you not host DNS with your registrar. And if possible, that you not host DNS with your hosting provider (although, that can be more difficult).
If you have a high value domain, you might want to look for a corporate registrar, like MarkMonitor or CSC, or anyone else who can do Registry locks (which are very different than registrar locks and are rather inconvenient, but potentially very useful); but know it's going to be expensive. I also had a good corporate experience with register.eu, they've got a lot of ability to satisfy foreign presence needs for restricted TLDs, if that's something you need/want. If it's a low value domain (like my personal domains), I don't have strong feelings, except for the love of whatever you hold dear, don't use Network Solutions; they were a fine choice when they were the only choice, but ever since we had options, they should have been used. A lot of registrars are really pushy with upsells and what not, so I've tried to go with no fuss registrars over the years.
In terms of DNS services, I don't have any particular recommendations; I personally run my primary DNS on my hosted machine and secondary with Hurricane Electric, which is free for my usage. There are (or were) several free secondary DNS services out there, but the one I used to use stopped maintaining their website (TLS 1.0 only, certificate issued 2014, expired 2015) and I already had an account with HE's tunnel broker, so it seemed like a reasonable choice. I still have a domain I host for a friend that uses that old service, because I can't get my friend to update the glue records at her registrar; the service still works enough, I guess.
I thought your original comment made it sound like there was some obvious reason for keeping DNS and registrars separate.