The bolded point above by Vitalik is that if transactions are free up to the burst limit, then the DoS attackers can avail of it. Thus the DPoS blockchain is attacked and not just the servers (nodes) on the periphery. A sock puppet attack can be employed to defeat any attempt by the block producers to fairly limit each user. This would have the effect of forcing all users to the minimum transaction bandwidth their stake will accord, because a significant portion of the stake is maxing out the bandwidth and compensation of the block producers.
The burst capacity is of limited use to spammers. There is a burst limit that is some moderate multiple of your minimum allocation, I think currently 10x. If you have 0.1% of the stake then you can use up to 1% of the bandwidth, burst capacity permitting. Even if you spam at the max rate, that still leaves 99% available for others to share. If the blocks start to fill up then the overcommit ratio drops. A 0.1% stakeholder would then only be able to fill up hypothetically 0.5% if the overcommit ratio were cut in half. (I don't know the exact algorithm governing it.)
You would need to own a meaningfully large portion of the stake (10% or more) to force users to burst at their minimum allowance, and even then the overcommit ratio would dynamically drop and then you would only effectively limit them to a smaller multiple of their minimum allowance.