I disagree with the basic premise here - that Dumbledore was fetching Snape to have himself killed. I don't think that was the plan at all, for several reasons (I don't have the book with me so no quotations, but if you've got any throw em in!)
1) Dumbledore didn't "plan" his death in any detail - it was something to be done as a last resort, to spare him from Death Eaters, or to protect Malfoy (and his hand obviously). It was NOT something that was simply to be done on a whim whenever Death Eaters were around, but a final gambit to be used when there were no other options left.
2) Dumbledore was greatly weakened by the potion. Remember, the trap was designed to completely incapacitate a wizard, allowing either the Inferi to drag them under or Voldemort to return to investigate. It's doubtful that any wizard other than Dumbledore could have performed magic AT ALL after drinking the potion, never mind the powerful spells he used.
3) Dumbledore would not have left the students to the mercy of the Death Eaters. His death could be carried out in a myriad of other ways that didn't involve Greyback stalking the halls of Hogwarts.
So why, then, was Harry instructed to fetch Snape on their return? Simple - for the same reason Dumbledore ran straight to Snape the LAST time he returned to the castle, gravely wounded by Dark Magic after investigating a Horcux. He needed Snape - the only man in Hogwarts he truly trusts - to help heal him.
Even more than that, Snape is his inside man - if the Death Eaters are attacking Hogwarts, Snape is the most useful source of information in the castle, the BEST way to figure out what's happening and how to proceed.
But then, of course, the weakened Dumbledore senses Malfoys arrival and realises that this is no longer an option. This has changed EVERYTHING. Malfoy is on the Death Eaters side - if he has arrived the others must be close behind. So what are his options?
Disarm Malfoy - we don't know he had the time to spin around in his weakened condition. Even if he had, he's risking starting a potentially lethal duel between Malfoy and Harry. AND if this is successful, he still has more Death Eaters approaching, who are older, fiercer, and more skilled (especially when they pass the cry that Harry Potter is in the tower!). It's unlikely that they can fight their way out. And if they DID, Malfoys life is forfeit.
So no, disarming Malfoy isn't an option for all sorts of reasons. The only thing he can do is hide Harry and see how the situation plays itself out. This is why, as soon as Snape enters the scene, Dumbledore begs him to kill him. Best possible outcome, given the unfortunate circumstances. I think from the moment he Petrified Harry, he had accepted his likely fate, but not before.
NOTE - One of the other posters has suggested that Dumbledore wanted Harry to see his Death to tip him off about the Elder wand. But this wasn't Dumbledores plan. Dumbledore intended the wand to die with him (since he asked Snape to kill him the wand wouldn't count this as a "defeat"), allowing it's power to be broken. This is another indicator that what happened in the Tower was Dumbledore being forced to think on his feet.