This is true, but doesn't have to be the case.
I spent about 30 minutes setting up my article using The Best Spinner. I spun it a few times and wasn't completely satisfied with the results. As you said, it was sometimes hard to read.
Since my articles currently bring me most of my traffic, I decided that it would be smart to spend more time spinning them and checking the results. I put about 2 hours into spinning the title, summary, article, a resource box, and keywords. During that time, I constantly checked the spun articles and "tweaked" the spin text. I wanted to eliminate EVERY grammatical error I found and make it very unlikely that anyone would ever think this article wasn't generated from scratch (i.e., not spun).
After about 3 hours total, I am able to generate 750 word articles that:
1) Contain unique content
2) Are grammatically correct
3) Are well-written and are easy to read and understand.
4) Appear to be individually proofread.
Of course, this 3 hours and this is probably way more than most people would spend on one article. It seemed worth it to me because I have posted about 300 spun versions of it all over the place and articles bring me the traffic that search engines don't.