[quote="EdtheHyena"]But Scar had already warned Mufasa in time. From what I recall, all Zazu did was tell Simba his father was coming.
[/quote]
Except that you are forgetting a very important part.
Scar only mentioned that there is a stampede in the gorge, and that Simba is down there too. In terms of Mufasa actually finding his son on that information, it's rather useless, as the gorge is a rather large canyon to look for a small lion cub in the middle of a stampede made up of easily thousands of animals much larger than Simba. This is not even taking all the dust into account.
Zazu actually found Simba and immediately relayed his exact position to Mufasa: "There! There! On that tree!" If Zazu didn't show Mufasa exactly which tree Simba was on, the cub would have been history.