Some of the posts here make it sound like RhythmAuthors are the only ones offering authoring services for no up-front cost. All the major RBN authoring groups offer various arrangements ranging from paying a one-time fee and all the royalties going to the band, to paying nothing up-front and taking a cut of the royalties.
Either way, it's still going to cost the band quite a bit, whether it's up front or taken out of the royalties over time. The royalty option is obviously very popular for indie bands who don't have the cash up-front and don't know how well their songs are going to sell. Likewise, it's a risk for the authoring company to take on a band on a strictly royalty-based arrangement if that band is only going to sell 1000 songs at $1.
The reason it's "worth it" is, well, if you tried to author an entire song yourself you'd know. Someone trying to author a song on their own could easily spend 100+ hours by the time it gets through Peer Review. An authoring company can have a song authored in a very short amount of time because they're splitting task between people and those people are intimately familiar with the process (as opposed to a band trying to learn everything for the first time).