For me, quoting practices like these are simply unacceptable. Unless you're doing some sort of satire, there are no acceptable (or honest) reasons to hack and slash the source's original thoughts to pieces just so you can reassemble them in a way that suits your purposes. Certainly not without pointing out what you have done.
Even the partial quote thing is, to me, way too slippery unless you clearly point out that you are quoting the half of the statement that agrees with you, then go ahead and quote the rest.
If you don't point out what the full thought was, you are allowing the reader to think - because you implied it by omission - that the source agrees with you in full. This lends weight to your argument in the reader's mind; a weight that your argument doesn't deserve. As far as I am concerned, if you do this, you are cheating. ON PURPOSE.
To me, this simply rings as an excuse and comes off as really underhanded.
Also, putting the burden on the reader to fact you just isn't good enough. You're the author. Fact checking your work is your job, not mine. If I need to go to a library to check your facts and discover that your book isn't half as convincing as it seems, then you haven't done your job. Also, again, that's really underhanded. You gave me a presentation that made your work seem more convincing than it really was and used this as a lame excuse in case you got caught. I won't trust anything else you write ever again.
To me, these are slippery excuses for twisting the truth to suit one's purpose. Nothing more. I remember a book by Al Franken - I think it was the Lying Liars book - that sort of lampooned the absurdity of claims like these.
You can make mistakes, but even then, you own up to them once you realize what you've done.
