I still think they're far from perfect (ditch the funny angle in any 21st-century bus redesigns and make them nice, crisp LED screens rather than vinyl) but I'm definitely coming round to their place in a bus company's media mix.
Note the use of bus company and not any other type of company.
Your audience is captive and has already purchased one of your products, so why not introduce them to other services/routes/tickets they could take advantage of? You couldn't write a better cross-selling opportunity script.
The trick is not to overdo the messaging or overdo the branding. Too many messages and none of them get noticed, repeat the corporate colour palette cove after cove and the eye begins to fight against what it sees.
It's a subtle balance but one, with a little finesse and thought, that can work wonders for brand awareness and revenue generation.