Actually, these kind of frequent fliers do implicitly get something extra and rather valuable out of BA. They're the ones who are likely to know which are the good seats and bother to grab them at exactly T-24. Before I was a Silver (only a couple of years ago) I was virtually always able to get an exit row at T-24 when in ET, which being tall meant a lot to me. Some people simply don't care about this (so would hardly take advantage of Blue+ anyway) and people who are only occasional BA travellers often don't have the hang of how OLCI works and are unlikely to make sure they are at their computer at exactly T-24; and even if they were, they are easily deterred if they're a minute too early and may not try again until the next day (this has happened with friends of mine).

This isn't an argument against a Blue+ tier. Rather, I'm just pointing out that OLCI already works in favour of the moderately frequent traveller who cares about where he sits.