The US constitution provides for separation of church and state. Their government is restricted from promoting, endorsing or funding groups simply because of their religious beliefs. US citizens are among the most religious people in the world This is based on the principle that whereas preferential treatment of religious beliefs by the state (state accommodation of religion) emphasises difference by treating specific groups differently, separation provides fairness by promoting equal opportunity for all to practice their Beliefs. This separation of church and state enhances the diversity of belief in American life.
This is just one of the reasons for having a separation of church and state in Australia.
Favouring of religious or other Belief is unfair, as the favouring of one religion over others is arbitrary and divisive. Separation would avoid the inevitable unfairness arising from discretionary government decision-making regarding eligibility for favourable (or unfavourable) treatment of disparate, and possibly contradictory, religious organisations.
State-Belief separation avoids equating personal and/or religious moral principles with society-wide, political moral principles. The former are personal, and the business of the individual. The latter are the legitimate concern of government in formulating prescriptive policies for everyone. They involve principles of public political culture. As they are subject to an overlapping consensus on the principles of democracy, they remove the unfair influence of historical connotations, preconceptions, biases and political manipulations contained in many current approaches to religious belief, including the pressure for prima facie exemption of religious practices from the rule of law, including criminal law.
Where there is state-Belief separation, the state is prevented from imposing any practice or prohibition based on religious doctrine, surely an unfair practice that can result from accommodationism.
Unfair financial and political influence is avoided by state-Belief separation. State-belief separation means religion and other Belief are above politics. Government resources (taxpayer dollars) would not be applied to activities that promote particular religious beliefs or activities. Tax exemptions for religion (other than for charitable works) means millions of dollars are lost to state revenue for pressing social services. Funding of religious activities also amounts to millions of dollars. Government is not funding conflicting moral interests.
Separation from Belief prevents state interference in religious or other practices. Government accommodation of religious activity is often subject to specific conditions as to where and how it may be carried out. This control can compromise the tenets of the religious body, result in division within the religious community, divert church resources to fulfil government requirements, and even force changed religious tenets or practices. The danger of state-established religion to religious minorities has also been well recognised.
Finally, state-Belief separation avoids marginalisation of non-members of favoured religions, and consequent denial of protection or privilege. This denial can result in resentment and social disharmony. With separation, only religious activity that is contrary to the welfare of others is restricted. Separation is overall socially inclusive, not exclusive.
 Evans and Thomas, 'Church-State Relations' 713. See,also e.g., Tushnet, 'Questioning the value of Accommodating Religion'; Ira Lupu, 'Reconstructing the Establishment Clause: The Case Against Discretionary Accommodation of Religion' (1991) 140 University of Pennsylvania Law Review555esp 582ff.; Smith, 'The Tenuous Case for Conscience'; Marshall, 'The Other Side of Religion'.