The day itself is calmer
Integrated pages pay off most on the event day, when nobody has time to reconcile anything. Because the door greets guests from the same list that tracked the replies, you can see arrivals tick up in real time without a clipboard and a helper squinting at handwriting.
That continuity is the argument for keeping the plan and the count together. Marta and Tom are not managing a page, a spreadsheet, and a guest list on three phones. One link carries the invitation, the answers, and the arrivals, and it reads the same to the guest and to the host.