Not quite sure what you mean Ray, there is nothing to stop MPBA members participating in as many branches of the hobby as they wish but obviously at individual events you are unlikely to be able to combine r/c scale with tethered hydros for example. Model Yachting has always been separately administered as it evolved down a different route.
Although there are 'general' modellers such as yourself, I think most people tend to develop an interest in particular categories of models and even subdivisions within those categories such as warships for example. However the only barriers are those that people erect for themselves!
There has always been a bit of an attitude of 'what does the MPBA do for me?' And depending on your particular interests it might be 'not much' or 'quite a lot'. But the 'MPBA' is not an entity in its own right. It is just people who are members of model boating clubs holding national positions to help take a wider view of the hobby. I am not very active at the pondside these days but I still keep up my membership because I do think it is important to have a degree of national representation to help safeguard our hobby collectively and also when interacting with other organisations and authorities. If more people were willing to accept a role beyond just involvement with their local clubs then it might be possible to develop our hobby further in a positive way.
The old r/c navigation events held under MPBA auspices gave the opportunity to actively navigate your model round interesting courses which were great fun and to me, added an extra dimension to my modelling. I would like to see more of that rather than today's tendency just to sail round the pond in aimless circles which I find rather boring. We don't have to reintroduce the strong competition of the 70s & 80s but driving your boat around a fair but challenging course and carrying out realistic docking actions to me really does enhance the pondside experience and I think it makes it more attractive to spectators as well.
Colin