There is spam concern because there are transactional limits to everything. If there is no spam prevention, anyone can attack the network with say 100,000 false transactions and grind the network to a halt.
Okay, then use a cryptocurrency to prevent spam. In fact, you could use Bitcoin. The way to prevent spam is simple, which is attach a cost to an electronic action. Spammers are successful only when their cost to spam is negligible.
XRP is a crypto-currency.
Using any other crypto currency for that purpose would require Ripple to juggle with multiple chains, would increase significantly the load on that chain, create a dependency between Ripple and a currency that may not necessarily be future-proof due to the high energy cost of proof-of-work mining, and serve no purpose since Ripple already has a very efficient distributed ledger where it can handle it's own native math-based currency.
Among all crypto-currencies, Bitcoin is probably the worse fit as fees are orders of magnitude higher than current transaction costs in Ripple, and scalability is very limited.
The original Ripple was brilliant IMO. I was a fan. That did what the current Ripple does and without any underlying currency.
If the original Ripple didn't need an underlying currency, Ripple Labs's product wouldn't be called "Ripple" today.