are you saying That Prefmark is useful to avoid calls going on anything else than bcch trx? Well, ok i call that a very specific situation 🙂 as mania said it might be Time to use bbh.
the initial access is done on bcch ts0 but it is so short that even with low rxlev it could go through. But call setup (sdcch) and call (tch) are long and require a good average lev. Freq diversity can help improving lev by fighting rhe fast fading.