I thought about what Rob said (in his private capacity, not as forum administrator) and I completely agree with him.
The press? The public? To H*** with them. This is not a state or royal event, it will be a private event. As such, there are only five individuals with the right to express opinons or make this decision.
William - who probably wants his step-mother there, if for no other reason than as a sign of respect to his father
Harry - the same as his brother
Chares - who must attend for all reasons Rob mentioned
Caimilla - who probably will be shaking in her boots that day but should attend anyway.
And of course....
Diana herself. Now here's the interesting one. What would Diana think were she able to express her opinion? She had moved on with her life, making her own plans, happy to be free. I, for one, believe that had she not died, Diana by now would have resigned herself to the Charles / Camilla marriage (it probably would have happened earlier than it did had Diana lived). She may even would have held some comfort in the fact that someone she loved / cared deeply for, was now as happy in his personal life as she was in hers.
So if I'm right, the only five people who's opinion should matter would probably all vote "yes". Who can argue with that?