I thought other reasons for SI2 and SI5 is that as SI2 collated cell inform at switch ON and the MS at early stage in idle mode the network may not know MS there, the MS is just identifying the best cell on which to camp.
For SI5 the network has taken into account all cells within 35Km (?) boundary that could possibly be used by mobile but cell assigned for use is best on lots of factors taken into account eg:
other cells at traffic capacity
other cells with faults
other cells may have high interference
which the MS in idle mode, monitoring SI2, could not posibly know?