CELL ID decoding is very much time consuming. In order to read Cell ID from BCCH MS needs to adjust its frequency from FCH, to lock its phase by means of extended training sequence contained in SCH. After that MS is able read System information messages from BCCH. System information is not sent as often as SCH. In dedicated mode ,as you definetly know, MS has only 3,9 ms between successive TDMA slots and there is a lot of job to be done for the mobile within this time period. I think that is the reason for using the combination of BSIC and ARFCN for cell identification.