It would seem that Voldemort's decision to come to the Ministry was related to the loss of the prophecy. Note that when Harry is talking to Bellatrix at the beginning of Chapter Thirty-Six, he, Harry, 'feels' Voldemort's reaction to the prophecy being destroyed (my emphasis):
“Potter, I am going to give you one chance!” shouted
Bellatrix. “Give me the prophecy — roll it out toward
me now — and I may spare your life!”
“Well, you’re going to have to kill me, because it’s
gone!” Harry roared — and as he shouted it, pain
seared across his forehead. His scar was on fire
again, and he felt a surge of fury that was quite
unconnected with his own rage. “And he knows!” said
Harry with a mad laugh to match Bellatrix’s own.
“Your dear old mate Voldemort knows it’s gone! He’s
not going to be happy with you, is he?”
Moments later Voldemort turns up:
“Don’t waste your breath!” yelled Harry, his eyes
screwed up against the pain in his scar, now more
terrible than ever. “He can’t hear you from here!”
“Can’t I, Potter?” said a high, cold voice.
Harry opened his eyes.
Tall, thin, and black-hooded, his terrible snakelike
face white and gaunt, his scarlet, slit-pupiled eyes
staring... Lord Voldemort had appeared in the middle
of the hall, his wand pointing at Harry who stood
frozen, quite unable to move.
This does not seem to be a coincidence. Apparently, upon learning of the prophecy's destruction Voldemort changed his plan. We can perhaps find a clue as to what this was from what follows:
“Be quiet, Bella,” said Voldemort dangerously. “I shall
deal with you in a moment. Do you think I have
entered the Ministry of Magic to hear your sniveling
Here Voldemort says what is not the reason he came. This tells us that there is something else he came to do. And the very next thing he does is try to kill Harry:
“I have nothing more to say to you, Potter,” he said
quietly. “You have irked me too often, for too long.
So it seems that his new plan was simply to get rid of Harry once and for all. At this point his burning desire to eradicate Harry apparently overcame his caution. Additionally, he planned on doing a hit-and-run style attack, as is evident from his conversation with Dumbledore:
“It was foolish to come here tonight, Tom,” said
Dumbledore calmly. “The Aurors are on their way —”
“By which time I shall be gone, and you dead!” spat
Voldemort. He sent another Killing Curse at
Dumbledore but missed, instead hitting the security
guards desk, which burst into flame.
Thus, he may not have been too worried about being spotted (and/or apprehended) in the few moments he planned on being there. And he may also have been been under the impression that the Death Eaters had secured the Ministry premises, so that might have removed some of his caution as well. Plus, he wouldn't necessarily have another easy opportunity to attack Harry.