Hiya, :)
Yes, I am currently acting as P.O.A. for an elderly parent. We organised it a few years ago, even though the person I am 'Managing' is still lucid - and reasonably able to move around.
It comes in very handy, when they have to 'interface ' with the modern world for financial and medical matters, as of course you can do the I.T. work, wade through the documentation, assess risks - and generally take the stress out of many things for them. For us , it was not about imminent emergency, but rather allaying anxiety well before the 'End game' and thereby increasing their quality of life in general.
I was lucky, in that when I did it, it was only about £100 in fees to a local Solicitor. I gather it now involves a bit more effort - as it is a small Court matter ( ? ). I would say get it done, and soon - as the slowest part is getting the sometimes inefectual counter / battery hen staff at the Banks to process it through the system ( many don't realise it's gravity ) - and get your name printed as P.O.A on all issued documentation. Sooner is also better , because staff at the local Chemist / Surgery / Hospital / Bank, get used to seeing you with or instead of the person you are dealing on behalf of. Thankfully, they all start off cautious - and the process of real trust and co-operation from the officials grows some time after the documents get stamped.
I also feel that it will help when you have to carry out the seperate ( and somewhat painful ) proceedure of dealing with their estate , when they are gone; not because it carries over legally, but because you are already known personally to many people who's professional assistance you will require at that time.
All in all , a great idea - and one that also lets you show your love and gratitude to someone, by visibly doing for them, what they had long since done for you.
* Edit*.
Forgive me for mentioning the obvious, but the P.O.A ceases when the person dies ; so while you are at the office of the Legal Beagle, ensure you ( or your favourite family member ) apply to be made executor of the estate / get a will written by the the current ' dependent' . When one realises how much power the state has over an undocumented estate, it's ALL a good idea, it's ALL essential to make sure things don't get taken out of your hands at a later stage.