Firstly the state would save money.
Secondly the state should be entirely secular, if only to remain neutral towards all the different faiths that now exist in this country. If parents wish to offer religious education to their children, let them find funding for it themselves.
Thirdly religion in is not a good thing. It is highly divisive and causes more problems than it is worth. For example, why should the state fund something that ultimately is at the source of the Northern Irish Troubles, which haven't gone away yet, or of persisting sectarianism in Scotland, even in these so-called enlightened times? How much money are we spending as we speak on combatting religion-based terrorism, here and abroad? Let's save money and not add fuel to the fire at the same time.