Many companies provide domain registration services as an additional one. They are not domain registrars but still provide this service.
Let's take, Lunarpages.com, Site5.com and ServerPoint.com. This service isn't their basic one but is just mentioned on their web sites.
Do you think it's wise to use domain registration from them?

No, especially if your tied up in some of there other services apart from domain registration. ESPECIALLY HOSTING, your pretty much screwed if you want to move hosting companies forever. They have been known to refuse domain transfers... A LOT. People get in a lot of hassle because of these so if i was you, i would stay away.

I've had good experiences with what your asking, I use 3essentials domain services to register my domains, which in turn shows wild west domains as the registrar, which is a Go Daddy reseller. To date I've had no problems with transfering or pusing domain names to my own account or transfer\push to another persons.

Why not to reg your domain with the company you are going to host your web site. Lunarpages.com, ServerPoint.com and Site5.com seem reliable companies that provide their web hosting services for a long while. Much depends on the company itself but these ones are quite reputable.

It is a good practice to check the TOS carefully before signup to ensure the domain name is not tie down to the hosting account. Generally, I do recommend to use a separate domain registration service for domain registration and then signup hosting account with the web host of your choice.

Domain name is an important asset, you can loose a hosting account and recent content but you can quickly rebuild the website using a backup at a new hosting provider. However, loosing your domain name simply means you have lost all reference links to the domain and bookmark traffic of your loyal visitors, you will have to start fresh with a new domain again.

Before you sign up with the host, you better read the TOS and talk to them abt this, coz some people charge around $10-15 if you want to make a domain transfer

Domain registration service is used an additional one by many companies. There is nothing new in it. The companies you have mentioned (LunarPages.com, ServerPoint.com, Site5.com) use it as well. I think if you use it, you'll lose nothing. It's rather cheaper and more convenient to use domain registration along with other web hosting services IMO.