One issue of RREQ broadcast message is creating what's called broadcast storm http://www.cse.fau.edu/~jie/teaching/fall_2005_files/broadcaststorm.pdf
due to the propagation of RREQ throughout the network (due to their nature as broadcast messages) the frequent transmission of those RREQs can contend with the transmission of data messages which will degrade the throughput of the network.