[adapted from my comment on a FB post]
Benedictions don't occur in vacuums. A convention is a politically charged event. And a benediction is explicitly understood to be a religious event. I understand why many religious people like the idea that their government endorses their belief system but the fact remains that the Constitution forbids such endorsements. And a benediction closing out a political convention is one such endorsement.
The counterargument is that the particular benedictions given at conventions, like city council and congressional invocations, are non-sectarian in nature, meaning that no explicit endorsement is being provided to any one group.
My response to that counterargument is that (a) any mention of God is necessarily sectarian because it naturally excludes non-believers and believers of non-monotheistic faiths; and (b) any such attempts at non-sectarian religious gestures in a political setting necessarily degrades "religion" by in fact adopting a watered-down, state-sponsored religious artifact (sometimes referred to as "civic religion").
The counterargument to my point (a) above is that America has a monotheistic religious heritage that deserves acknowledgement. Which would be fine if it weren't so easily shown that the true purpose behind the plethora of laws relying on this argument (ascertainable if you dig around the facts of litigated cases, as I've spent no small amount of time doing) is actually to advance particular sectarian agendas (think Ten Commandments plaques in courthouses). And, of course, it remains somewhat discriminatory–if you're not a member of the monotheistic club moments such as these serve as awkward reminders of your exclusion. But no one really cares because these outsiders comprise a meaningless constituency, demographically speaking.
The establishment clause was made possible because of an existing detente between the various competing religions in the mid to late 1700s. What we're now seeing is a melting of that detente, where the various religious groups have found common ground, i.e., civic religion, perhaps best thought of as the overlapping center of a Venn diagram containing all monotheistic religions. But civic religion is also memberless; a creation of compromise which cheapens actual belief by bending opportunistic religious leaders toward it.