Hmm, they should have used BTC in denominations of "satoshi" for the spam problem, this would maybe allow ripple to gain adoption and people would focus on the trust system more.
If you're referring to something like this:
https://en.bitcoin.it/wiki/Alternative_chain#Paying_for_resources_on_alternative_chains_with_Bitcoinstheir system would rely on the ledger consensus AND ALSO on bitcoin's chain for security, it would be far more complex, the improved latency of the new system would be cancelled...
And all this apparently for no good reason.
If it's about the non-p2p issuance, the new system would still be better by just issuing the coins through proof of work as a one time only thing.
It turns out that when you imagine such a distribution mechanism without providing any security (like the issuance of bitcoin is providing to bitcoin or the 20% of freicoin's is providing to freicoin), it becomes more clear that is wasteful and there must be a better way. At least that happens to me, obviously other people here think different.
I see no reason why someone could not build a ripple trust system on top of BTC.
Actually you can. With colored coins you can build ripple transactions (with several "colors"), but for now there's only a prototype for simple "One color for BTC (uncolored coins)" transactions.
But colored coins rules are not enforced by the chain, only the amounts. Everything else is outside and must be made "manually".
For example, take this ripple graph:
Alice owes 10 to Bob
Bob owes 10 to Carol
Carol pays 10 to Alice
In Ripple, all get back to zero automatically, the ledger enforces that. In colored coins, although it can be made atomically too, Carol needs to explicitly send bob's 10 back to Bob, and bob needs to sign that he sends back to Alice her 10 IOUs too.
Apart from requiring a more complex implementation, it requires all parties involved to be online at the moment the transaction is created, with the ledger only the payer needs to be online.
Bitcoin would require a hard fork to do that. Since they're not forking Bitcoin, obviously they consider the ledger superior to the blockchain as a p2p consensus mechanism.
That doesn't mean Ripple can't help Bitcoin grow though.