what is the point of having centralised currency, I thought the plan was to move away from this
Exactly my thoughts; Ripple currency is bitcoin without everything that makes bitcoin great. Whether you'd call it a scam or not, it's pretty much not the way I want currency to go.
1) Ripple is a decentralized clearance system which facilitates the transfer of money from one entity to another. It compliments bitcoin and other currencies.
2) XRP, is a utility of the ripple network. 1 XRP is the fee for roughly 10-100k ledger entries. Each fee is destroyed, reducing the overall amount of XRP in existence.
Thus, if (1) succeeds and gains significant adoption (it's certainly growing) then you can calculate the long term value of 1 XRP as being the USD value you'd associate with 10-100k transactions / ripple ledger entries.
I personally hold multiple currencies, BTC and XRP are just two of them.
You can use ripple without needing to pay for XRP, just get some from the giveaway or ask somebody for a couple of hundred and you'll have enough for a lifetimes normal use of ripple without worrying about XRP the currency.