Should the server not provide an access token to the client if these parameters are in the request uri instead of the body?
This probably makes sense in order to educate programmers (if they don't get an access token, then they'll learn to do it the right way), but it wouldn't make the system more secure if it was under attack. If the client id and client secret were stolen, then the attacker could impersonate the client and get a valid access token by sending a correct request (with the id and secret in the message body).
What are the possible dangers of posting these parameters in the request uri?
If you use https to communicate with your endpoint and you trust your access token provider, the danger would probably be small.
The id and secret could also be stolen at the other end of the connection, since they show up in the web server logs of the server you send it to, but because you're sending your client credentials to a provider who already has them, and who hopefully knows how to keep his systems secure, I'd think this wasn't a likely point of compromise.
If you use unencrypted http, then the client id and secret might end up in logs of caches and proxies. Whether somone who sniffs your traffic can see the URL is only midly relevant; since in order to do that he has to look at the request, and therefore could also see the id and secret if they were sent in the request body.