Because the bitcoin traffic, if it keeps growing as it has over the last 12 months, will hit the effective capacity of the mining network by mid 2016.  Then traffic jams will be frequent, and the average time to 1-confirmation will jump from ~10 minutes to hours depending on the time and day.  Bitcoin adoption will stop growing and bitcoin's image will suffer.


The stress tests were not directly related to the blocksize issue.  The company that claimed responsibility for them (Coinwallet.eu) has some wallet software that, they claim, will compute the correct fees to get over traffic jams like the ones that they did.