It will make a big difference on routes where wide-bodies are already full. There are a bazillion of these in Asia, quite a few in Europe, fewer in North America - which is why, aside from regional rivalries, it's usually North Americans who don't see its value. That pattern also matches, roughly, who's buying it. Aside from the economies, many airports are landing-slot constrained, so larger aircraft are the only way to move more people in and out.

Aside from that, there are days when all those BA flights, as well as everyone else's, are full. Since it's not practical to switch aircraft the day before based on how many tickets you've sold, you have to fly something big enough for most days. That's an oversimplification, but the fact that 747s have some empty seats much of the time is not by itself a basis for concluding that an A380's capacity would be wasted.