What are the drawbacks (apart from much fewer reviews of newer items)?
Shill reviewers will then buy, review, and return the items, which will increase the retailer's cost of dealing with returns or and force them to either increase prices or institute more restrictive return policies that end up inconveniencing legitimate purchasers that have to do returns.
EDIT: another problem is that it prevents legitimate reviews from people who, e.g., receive the item as a gift, which might not seem like a big problem, but some items may be more frequently purchased as gifts than for use by the purchaser.
This wouldn't prevent shill reviewers to negatively review the concurrent items, but Amazon could easily identify an account that has a abnormally high return rate + negative reviews and forbid them to review.
(I guess requiring reviewers a minimum spending before reviewing would avoid shill reviewers to create an account only for the purchase)
"The suit also alleges Gentile told the customer that the seller could “simply ship empty packages in an effort to fool Amazon into believing the reviewer was a ‘verified purchaser.’ ”"
>The suit also alleges Gentile told the customer that the seller could “simply ship empty packages in an effort to fool Amazon into believing the reviewer was a ‘verified purchaser.’ ”