Agreed and if it really was a concern, one could run a script on the database that could alert the admins of the massive build up of characters comming from one subscriber account.This is sort of a weak explanation for a 20 character limit. First, one would presumably need to have a DDI account to start uploading characters with a credit card on file. But let's go ahead and say this hacker that want's to take WotC down gets his stolen credit card number, signs up for an account and then proceeds to run this malicious script from 20 computers simultaneously.
It looks like from other posts that a fair number of dnd4e files are around 200kb in size. The same information in a DB table is quite likely to be less than that, but we'll go with 200kb. So 20 machines running the same script for an hour will generate 1.4GB of data. Working from there, rounding up to 1.5GB per hour means it would take 682 hours to generate 1TB worth of files. And even if the size of file is double the 200kb I worked with you are still going to need 341 hours to fill 1TB. Or even if the script runs in half the time, one still needs a good number of hours to cause harm via this method being used to justify the 20 characte limit.
So a 20 character limit seems to do nothing to really prevent an overnight attack of this manner other than frustrate the legitimate users of this system. It seems limiting simultaneous logins would do more to circumvent a scripted attack as described in the above quote.
And finally, anyone that wants to attack the WotC DDI servers is going to find a more traditional DDoS attack on their network and servers much more expedient than trying to fill up their disk space by uploading a multitude of fake 20th level characters.
It seem a week reason and I suspect that the real motive is either they want to sell more slots later or there were concerned about the volume of data that would be uploaded when it goes live. I have 48 characters on this machine right now.
Still 20 characters at 200kB each by 41k users is about 150GB which is not alot