I personally prefer that everybody that does not train in Aikido believe it has no practical use. There are several good (at least in my opinion) reasons for this.
1. Such a belief would keep wanna be ass-kickers out of Aikido Dojo. I mean how many times have you had someone come into your home Dojo and try to prove how big and bad they are?
2. Hopefully if someone discounts the skills of an Aikidoka they not see a need to prove themselves against him/her. Alternately if they want to teach that Aikidoka a lesson about real life fighting the aggressor might underestimate the Aikidoka.
3. It makes that Nikkyo even more of a surprise. ^_^
Remember please all of the above is my opinion only. Who I could be (probably am) wrong.