There are arguments for and against. Protecting and reinvigorating Celtic languages, as all endangered languages, could be seen as cultural or linguistic ecology because when you lose a language you lose a lot more than just grammatical rules and dictionary definitions, and it is certainly something that I think we would all support. At the same time, the Welsh, Cornish and Breton spoken today would probably have been unintelligible to an Iron-Age druid and vice versa, just as Modern and Old English or Norwegian, Danish and Swedish and Old Norse would also be mutually unintelligible, Icelandic and Faroese perhaps the exceptions. I don’t think we should go down this “ethno-centric” route at all, in fact I think it’s a very unwise route to go down.
Moreover, all around the world people follow numerous religions, faiths and philosophies without necessarily using or knowing the “original” languages their scriptures were written in and still manage to have fulfilling and rewarding outlooks on life and experience and I feel that this is more what we should be about. At the end of the day, what language does an oak tree speak? You can revere the oak in any language you wish.
My own outlook is as follows: should someone who is following a bardic course and exploring druidry take an interest in the Celtic language? Yes. Must they have an obligatory advanced level in those languages? No.
Just as an aside, the closest language to Old Gaelic is Irish, not Welsh. Don’t forget that the Welsh, Cornish and Bretons would also they were Britons, i.e. British too.