It's pretty easy to see that not everyone will switch their main (I don't ever foresee leaving my main, although I do plan to level a Death Knight as an alt), but even if people just roll them to check it out, it reminds me a lot of the Star Wars Galaxies "new game enhancements" where they made Jedi a playable class. If you can roll a Jedi as a class, why would you roll anything else?
Of course, we could give Blizzard the benefit of the doubt here-- they haven't revealed much at all about the game's first Hero Class, so maybe it'll be so hard to get one that they really will be very rare, or they'll only be allowed in certain areas (so you won't see a pack of Death Knights swarming around the mailbox in Stormwind). We already know that they'll start at a higher level, so the good news is that you won't see Elwynn Forest flooded with a bunch of level 1 Death Knights. But as for other ways to keep what is supposed to be a special class special, we'll have to see what Blizzard comes up with.