The only defence I can think of is that it breaks authors out of a vicious cycle. A lot of reviewing / promo sites and reviewers will implement some sort of quality control - based on existing reviews.
I have seen it said that a book needs X amount of 4 star+ reviews before it will be considered.
For the author to get reviews, they need to increase publicity. To increase publicity, they need to get reviews. To get reviews, they need... and so on. It can be massively frustrating, and so the temptation is to put those bogus reviews up there, so that they can secure genuine reviews.
That's how I see it, anyway.
And, no. I haven't done any reviews of my own work - although I'm working hard to find other people willing to read and review!
If the only way to get a review is to do them yourself then your time would be better spent writing a better book. No one can tell anyone else whether or not they can publish anymore but once it's out there it's up to readers to decide whether it's good or not. If the only way a writer can get a review better than a one star is by writing it themselves then they probably shouldn't be publishing.
Sockpuppets, strawman sockpuppets, meatpuppets ... they've all made the review system on Amazon and other sites practically worthless. I rarely even read reviews anymore - what's the point?
I wish all the bookselling sites would do away with them. After all, there is a look inside and a seven-day return period, do readers really need them? There are plenty of outlets (blogs, newspapers, emagazines, etc.) that provide reviews for ebooks. Let those places be where readers go if they need an honest review, because it's hard to get many on Amazon. IMO
I like reading the reviews, although its the three star ones I usually pay attention to. I'm always suspicious of anything gushing and a quick glance at the look inside usually tells me whether the one stars are justified or not.