The biggest disadvantage is that you are adding more complexity to your database architecture. With more complexity...
comes a higher cost in maintaining and administering the database along with a higher chance that something will go wrong, i.e. a bug can cause your problems.
The second biggest disadvantage is the cost associated with RAC. Oracle is touting RAC on Linux as a way to acheive cost savings over large Unix servers. With RAC, the costs shift from hardware to software as you need additional Oracle license fees. The big question is will this shifting of costs result in any cost savings. In some cases, yes, and in other cases, no.
If you do not need RAC, then I would suggest not even employing it. If you are looking for higher availability, higher performance, or to shift from high-end Unix servers to cheaper Linux servers, then RAC may just be the solution for you.