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, DOS attacks on these pools have already been a problem.
What if NSA (or another government agency in a country where BTCGuild resides) forces BTCGuild to comply with authority orders, which they are legally forced to keep undisclosed and lie to their users about, just as in Yahoo, Google, Facebook, etc. ? Does this pose a risk?
If the pools are in fact not a risk, then was the initial Bitcoin pitch about decentralization an exaggeration? We don't really care if Bitcoin is mostly centralized now?
Lets say the biggest pools are knocked out completely. Would users still want to run their miners solo, or in smaller pools?