I believe it's just been passed down from earlier development. It provides flexibility in case things change. A safer approach if you will...
In reality there can be multiple base and offsets but that only exists in a SZ OmniLink network. Each Zone (differentiated by system id) will have its own b/s/o. Look at the Aussie GRN network for example. Though each Zone/SysID has its own ZC and its own subscriber database. So you may or may not want to link OmniLink sites; some systems it won't matter as they'll config them to work together and not rely on tresspassing.
(For the anally retarted we're speaking strictly of 3600 here.)
-Wayne