I believe it has a benifit.....first of all it brings your Mach No. predicted by FMGC closer to your actual Mach number which reflects on you time and fuel predictions(Mach No. is directly affected by temp.)..... if you don't update it in the fuel pred page, the FMGC wil use the pre-inserted temp. in the wind/temp. page which might be inaccurate....you can always update that one instead....so its a matter of fine tuning your predictions in flight when you find a deviation from the forecast figures....
secondly and more importantly, updating your temp. in cruise affects both your OPTIMUM and your REC MAX altitude, which can have a great efffect on your flight with respect to ATC restrictions, routing and fuel savings(check your QRH tables for MAX/OPT alt.).