Will Sonar function correctly if I delete RXP player...
 
One question though ... is it useful?

If you use REX files, yes, it's VERY useful even though it's an oldie-but-goodie (or ahead of its time, depending on your outlook). Here's some info.
 
http://www.soundonsound.com/sos/mar06/articles/sonartech.htm
https://www.soundonsound.com/sos/mar09/articles/sonarworkshop_0309.htm
http://www.soundonsound.com/sos/mar15/articles/sonar-workshop-0315.htm
https://www.soundonsound.com/sos/nov08/articles/sonarworkshop_1108.htm
 
There's no real reason to delete it. You can just exclude it using plug-in manager if you think you might want to use it at some point in the future.
 

It's just a "synth" that plays and edits REX and other files. More of a sampler/looper I guess. Actually I'm just going through all the synths to give them a good test/workout and that one's coming up soon.
 
Why would you want to delete it though? It ain't hurting anything. Just another tool.

Ok... will read and learn!