Given the state of the Piccadilly line (Hot, with All The Stairs, and All The Suitcases), I'd have Heathrow Expressed it too. Gatwick is different because it runs its Express on the same line as the local stopping service, so all you gain for the price is time. The Heathrow Express cuts out the Tube/bus options entirely, and is worth it even without the huge suitcase.

TL;DR: Ticket guy was an idiot.

H