I'll adjust that to just about worth having...
A £10 ticket certainly isn't worth the cost of running the bus from the terminal and employing an extra quartermaster to man the gangway. Add in the revenue on duty free and it probably just about is. Won't happen on the Dunkirk route, as those ships aren't fitted with a deck 5 walkway, but the Calais ones all are at present - clearly Dover turnaround times don't allow for boarding foot pax via the car deck.
It's marginal, but frankly the whole ferry market is pretty marginal, and has always been structured like this to take advantage of any extra profit centres on top of the main business. In the old days, rail pax were the bread and butter, with cars an add on, now freight is king and even cars are a marginal top up once the cost of going from 12pax to a full passenger certificate is added in.
--------
(edit, not sure why these two posts merged, below is a separate but related thought...)
One unexpected casualty will certainly be the cruise industry in Guernsey. It's a lovely place but a horrendously difficult port of call, and there is no doubt that 90% of calls are only to give cruises duty free status. DF is a fairly big deal for Cruise lines, not particularly for 'off licence' sales, but on board bar sales are big, and taxation is awkward and expensive. Anyone in doubt of this importance should look at how fast certain lines returned to Tunis after security incidents! Ports in France, NL and Belgium will certainly replace Guernsey as the duty free destination for short cruises and British Isles cruises.
On the plus side, I would expect to see a significant increase on cruises to Scotland in particular from Germany, at the expense of Norway.