This "limited number" is about 80% or even more of all Ripple nodes. This number is already RIGHT NOW higher than the number of servers you need to take over to severely mess with Bitcoin (which is about 2-3 pool servers).
Debate away!
none of the pools are at 51% even. to do this "attack" at the moment not only will you have to take out 2 or more pools with estimated 15% of network you would also have to take over btcguild at the same time. can it be done? yes is it likely to be done? no. the chances of it being done are very improbable. i would also assume that this would get noticed fast and miners would just point to other pools for time being
Of course, there is also the issue of whether bitcoiners should simply trust the entire security of Bitcoin to just a few operators.
Lets say BTCGuild knocks out 2 of its biggest competitors, then 51% attacks the network... all in collusion with or under pressure from the government. Sure maybe we could recover from this, but it would be a major hit to public confidence in bitcoin.