While I don't see it as workable for world domination (among other things, I don't think the changes to attitude would 'stack' as the source is the same), it's still a fairly powerful 1st level spell for someone willing to make use of it.
First, use Diplomacy to get someone's attitude as good as you can make it, then use this spell to modify attitude for whatever is appropriate for the person involved
-For a local merchant, "How about a discount?" is probably not beyond the pale, and if they are already helpful, or indebted to you, you can likely push whatever discount you would otherwise have gotten significantly higher.
-Similarly, getting an understanding of the local criminal goings on from a low level (or even mid-level) criminal who is now super-helpful, or an idea of what the city guard is up to from a super-helpful guard...these are not minor advantages.
The key I think is not to overreach, but to develop a significant network of 'enhanced friends'.