Well, the benefits in that regard are that (1) you don't have to come up with the stuff on the spur of the moment and (2) the Core Classes, Prestige Classes, new Magic System, and so forth aren't things that one would produce on the fly, being designed and playtested by folks very familiar with the d20 Engine. In addition, it's a superior product over
Naughty & Dice, which is almost all fluff with a poorly designed d20 section written by a couple of GURPS designers with little-to-no experience with 3E/d20, and presented in a much more mature package than
GUNK or
Nymphology. Add to this an active fan-base that openly share material related to it at the Valar website (nearly 1700 members to a message board for only one product) and a Mail List.
(All of which, btw, would seem to indicate that the material isn't particularly obscure; hell, the bloody thing's preview sold out at GenCon Indy and two shipments of the final product sold out at GenCon SoCal!)
Overall, there are now an abundance of new Classes, Prestige Classes, Skills, Feats, Spells, and Items Magical and Mundane, all of which are to make romance, love, sexual relations, STDs, seduction, and even chastity/celebacy have some effect on the outcome of a scenario and/or plotline outside of an on-the-fly decision of the GM. After all, there are
many players out there that cry without end when they disagree with a judgement call of their GM no matter how minor (or correct) the call was because it isn't a quantified mechanic and thus they feel that they can whine like babies until the GM changes his mind. Yet, these same players will gladly accept the result of a defined rule, no matter how horrible the result is, simply because it's a written rule. (

)
So, it's more of a set of questions you must ask yourself...
1. Is such material appropriate for your game and your group.
No: Don't worry about it.
Yes: Proceed to Question 2.
2. Do you want a rules set for sexuality that is written for mature Gamers?
No: Get
GUNK (Free) or
Nymphology (Cheap PDF).
Yes: Proceed to Question 3.
3. Do you want a rules set that is well designed for d20 mechanics?
No: Get
Naughty & Dice.
Yes: Proceed to Question 4.
4. Are you willing to spend the money for these rules?
No: Write them yourself or make judgement calls on-the-fly.
Yes: Get the
Book of Erotic Fantasy.