I don't know that the Press will cover the engagement as fully as they would have if Kate was there, TLLK (clothes, hat, hair.)
If, God forbid and please let it not be so, the Queen were to die tomorrow then as Duke and Duchess of Cornwall and later Prince and Princess of Wales, William and Kate won't be able to pace their engagements. Just like their father in law their diaries will be full and that will include daily engagements, followed by evening ones followed by ones the next day as well.
After all, Kate isn't being asked to run a marathon then follow it next day by a speech at the UN. These are two reasonably easy and pleasant Royal engagements, the former meaning a great deal to the Irish Guards. Kate should be honoured to visit such a famed regiment with a great history, not the other way round.
Then IMHO we're fortunate that the focus will be upon the Irish Guards and their mascot without the annoying photos accompanied by the estimated prices of Kate's hat, shoes and coat.