For a Street car or drag car no you don't need them. Pre 2005 sure you would want them in particular on convertables. I have them on y 90 GT vert.
Multimatic, Roush, and others use them on race only vehicles for serious competition and MUltimatic goes far and beyond sub frames welding cross webbing into the floor pan for a more rigid ride. I drove an FR500 GT at a driving school and the floor pan bracing was serious work.
For about $125,000 you can have a real bad ass road course racing machine. Ford Racing FR500C - 5.0 Mustang & Super Fords Magazine
2011 5.0 GT 3.55 gears, Candy Apple, Steeda CAI and SCT Tune,Steeda Control Arms, LCA Relocation Bracket, Pan Hard Bar and Brace, Ultra Light Springs, 3rd Link, Camber Plates, Tokico D-Spec Struts/Shocks, Shifter, Barton Shifter Bushing, Flowmaster hush power. Steeda 18X9 Chrome Pentar/Nitto NT05 275/40/18. BBK long tubes and OR xpipe
2005 F150 SXT, OX Locker, 4.10 gears, Edge CS Tuner, 33x10 AT, K&N FIPK, Gibson Exhaust.