I'm going to go out on a limb and express my opinion that the most important element of rumba is song. I have been in countless of situations where there were only percussionists and no singers, and while that is fun for a while, the percussion loses direction after a few minutes and it becomes time to change to a new rhythm. In contrast I have played a guaguanco part for what seemed like forever while singer after singer started up and the energy was maintained and the "feeling" was there.
One of the greatest aspects of rumba is it's participatory nature; if you know the part you can play. The number of instruments are limited, but the amount of people that can join in the choro is limitless, the more the better actually, each voice adding to the energy of the rumba.
Berta Jottar has some very wonderful videos where the voice completely replaces the instruments and also a nice video of rumba being sung a capella, with the only accompnaiment being clave played on a bottle.
Not only can rumba exist as song and voice solamente, but the singing makes the drums better. Without a singer to respond to and play around, the quinto can easily either lose inspiration and run out of things to play, or conversely the quinto can overplay and play over the other two drums.
The voice also gives the rumba direction as it progresses from diana, to the decima then the coro or rompe de la rumba, marking each section giving each one it's own feeling and flavor.
Singing is probably the most daunting skill to acquire in rumba. Not only are the songs in Spanish, a foreign language for many, there is the task of singing in clave or playing clave simultaneously. What makes a great rumba singer (gallo) is the ability to improvise in a call and response with the coro, a truly difficult skill. But take heart, the indomitable Barry Cox from New York has compiled and encyclopedic collection of rumba song lyrics. While it is nice to be able to improvise in Spanish beautiful and poetic lyrics, simply singing a well known song with an easy choro will go a long way.