Yes I think I remember they even used standard fees, at least for some tx

This "stress test" is, IMO, probably being done by a coalition of mining pools.  It exists for the plain and simple purpose of increasing mining profits by forcing users to pay higher fees.

Assuming they are X% of the hashing power, they are getting X% of the fees they put into it back when they get blocks.  So the expense is less than it appears to be.  They make a profit to cover the (100 - X)% because they ALSO get X% of the increased fees that users are putting in to cut ahead of their spam flood.

If we assume that legitimate users will pay increased fees for half the block space, and that the coalition represents more than 38% of the mining power, then spamming the network to keep blocks half full of spam (leaving the other half available for increased fee prices) is more profitable to them than not doing so.  

In practice it's even more profitable than that (or becomes profitable even for a coalition smaller than 38%) because (a) users outbidding them for block space are setting their fees to a nontrivial amount *above* the fee level of the spam rather than just equal to it, and (b) lots of users leave their fee settings higher even when the spammers spend a few weeks not supporting the increased cost of the "stress test".

I'm betting the people doing it DON'T want a switch to Bitcoin-XT; with the larger block size of Bitcoin-XT, legitimate traffic willing to pay the premium to cut ahead of the spam would be less than 1/2 the maximum block size and it wouldn't be profitable any more.

And you expect that they're being honest about why they're doing it.  I think that makes you what the cons used to call an "all-day sucker."

Bitcoin is quite popular and it may not change a lot for that.

No, that's incorrect.

XT miners have the same opportunities to collect fees as 1 MB miners, given a transaction rate that fills an equivalent proportion of a given block size. Is that not what this debate is about, increasing the carrying capacity?

You cannot make both arguments simultaneously; that tx rates will remain at present day levels post-fork and the miners will lose profitability, and yet also that transaction rates are set to soar which you suggest is not motivating miners at all? Are you making an honest argument Ray Dillinger, or did you just overlook that particular glaring contradiction?

Well that's why we need bigger blocks. I bet that stress tests that were going on lately have been and the future ones as well are here only to show us that the bigger blocks are the necessity.

The thing is also that we can't prevent these stress tests. People can be doing them as much as they want, all they need is some cash. And whole Bitcoin ecosystem is suffering during these stress tests.

Oh ok, I didn't know that! Can I get a link to learn more about it.
Thanks!

Down with stress tests spam attacks! 

I think this is a great point.  The cost to them is partially offset. 

Kind of sad that some segments of the bitcoin community would act so opportunistically against the group as a whole.  Normally, I am a fan of freedom and self interest but this seems counter productive in the long term as it reduces the attractiveness of using bitcoin and risks segmenting the community as we see with the XT-Core feud.

The "stress test" ceases to be profitable to a mining coalition if it costs them more to drive fees up than they get in increased fees.

Increasing the block size doesn't affect the opportunity to collect fees; it does however affect the amount of tx volume a coalition would have to pay fees for before it would have the effect of driving up costs for users and fee revenue for miners.

It increases the expense of the attack.