I'm actually not sure why they assumed the troll would be a distraction. The 3rd floor corridor is important, but IIRC it's not kept continuously guarded; the troll fiasco won't draw any guards away from their posts. Perhaps the thinking is that the troll might cause the professors to ignore the corridor wards, but I doubt they would be quite that stupid. And, of course, roving bands of professors searching for the troll on high security alert couldn't possibly be good for intruders, even if they're not specifically looking for them. Certainly, attempting a troll "distraction" seems far inferior to drugging Filch and sneaking in at 3AM.