Before asking question let me put here what I understood.
[ iBGP peers should not advertise routes learned from iBGP peers to other iBGP peers( as per BGP rules, which is there because AS-Path attribute functionality will be defeated..so this rule is good). The solution to this problem is Full mesh among iBGP peers OR Route Reflectors OR Confederations.]
We know lot of ISP's run IGP inside their core even though they have iBGP running. Why do they need this IGP(ospf or isis) if they have satified above required for iBGP.
My question is under what conditions IGP must be used while iBGP is serving well enough ?