I think (especially on Tripadvisor) it's not just the reviews that can be an issue.
I post on the TA Goa forum (as I do on here) and there are quite often blatent (in the opinions of many regulars) attempts to besmirch the reputations of various businesses, including jewellers, restaurants and hotels by rival owners posting on the Discussion forums.
I guess because TA is a more global website than HT and because HT
don't allow reviews from the country where the accommodation is situated & if an ip shows this is the case we remove the review and leave a message explaining why & asking the poster to resubmit on their return. We hope that in doing this we are discouraging rival owners from posting bad reviews, or, positive ones for themselves
spurious reviews are less of an issue with HT.
Regarding accommodation reviews, I find them useful but I think it's important to read between the lines.
I read a review of a hotel in Goa that we were thinking about booking and almost all the reviews were 8/10 or 9/10 then this one with 2/10.
Reading through the long review all the poster said about the hotel was 'it was clean, and the staff were friendly'.
Then followed a long rant about Candolim and Goa - from the red dust, taxi drivers, beach sellers to the cows in the road and the mosquitos!
None of which had anything at all to do with the hotel.
It was clear the poster hated his holiday overall.
I agree with merseygull about some people having hidden agendas, if a party has been chucked out of a hotel for anti-social behaviour and/or causing damage, they are unlikely to post a glowing review.
The trick is separating out the wheat from the chaff.