Why would anyone search outside Aikido for anything?
I guess that really depends on what a person is looking for. One might expect something from Aikido that Aikido is not/does not offer. One might 'only' be in the wrong lineage/style. One might have the right teacher, but not (yet) realise it.
There are so many reasons why people will not stick to Aikido in their search.
I think it really all starts by them (those searching outside Aikido, not the IS practisioners!) not really knowing what they are looking for.
In class I too sometimes compare what we do with other styles, but ALWAYS say it is not correct for us to it that way, never that they are doing it wrong only different (immediately followed by: "depends what you are looking for"). Students watch vids on youtube and have questions, rightfully so. I explain, show the differences. As for IS I am not so sure. Some have referred to vids of Ueshiba that show 'his' IS. All I see is good posture, good ma ai, kimusubi, kino nagare, etc. But that is just me, no judgement here (again to each his own).
Writers are always told to read widely. Read the genres and authors you like, but also the ones you don't, if only enough to understand why you don't like them.
very good point...everyone has something useful to tell, even the ones you do not 'like'. Aikido sharpens your judgement.