Not bad form as such, indeed it allows you to walk where road pedal cyclists fear to tread. That said, it might mark you out as a nouveau arriveste and, for a gentleman, that will simply not do.
Slightly more seriously, I guess it depends on what you intend to do on the road bike and how many of them you have. If it's just the one and it's main use is commuting or generally keeping fit then there's not much point in getting separate pedals and shoes. However if you have a fleet of road bikes and regularly ride sportives then it makes more sense (plus if you are doing longer road rides the extra sizeor road pedals can spread the load across your foor better.
Despite all the above, and the number of road bikes I have, I rode the Fred Whitton last year on mountain bike SPDs. But Paris Roubaix on road Looks. Go figure (although I think the answer is that it really, really doesn't matter).
One thing that might be worth avoiding is the baggy shorts and freeride jersey on a road bike for anything but the shortest trips, indeed if trips are so brief then jungle shorts, some sandals and a t-shirt are to be preferred.
