I'm writing a fanfiction on my blog about this. Usually it's quite easy to use on strangers that she may not see again and doesn't care about and may be suspicious of. However, once the strangeness and suspicion fades using the echo become impossible. This is not official of course, but it explains why she doesn't use the Echo on every NPC (such as Louisoux) who claims to have all the answers to everything.
Having an official answer on this would be nice. But... plot holes are easy to spot when someone else is reading your work.So I don't expect to get an answer.