The only possible damage it may cause is to blow the regulator/rectifier and alternator. If the battery is dead, the bike will be constantly trying to charge it which can cause the reg/rec to overheat and blow and it's quite an expensive repair bill. If I were you, I'd try charging the battery on a 12v charger that has a trickle charge. If you don't have one, I'd recommend you look at getting a ctek charging system, as it can revive sulphated batteries and can be left attached indefinitely and help to maintain the battery. If after charging the battery it looses it's charge again I would replace it, for the sake of £40 (typical cost) I would say it's highly worth it