There are various legal protections available (although similar protection is also available from other sources) which are invaluable in case of an incident (or your company turning on you).
You don't say which company you work for (as is your prerogative). Suffice to say, if BALPA is the recognised negotiator, it can be useful simply to hear what's going on with regard to T&Cs and/or pay deals, etc.
I suppose the fundamental question is this. How much is your licence worth to you? For £400pa, I would think of it as an extra layer of insurance.