They don’t need deadmines tested, it’s probably one of the most tested dungeons in the closed Beta. People are still running it in the closed beta.
It’s there as an inducement for people to try it at a more challenging level (level 15), and gather further metrics regarding player interest in Classic.
They might also throw some other unannounced(or soon to be announced) rewards to those who do the 1 to 15 grind on the stress test. There is precedent for them doing such things in the past.