To be quite honest, I like it when a product has a ton of reviews on YouTube. Some reviewers/gurus actually say on their profiles that they will NOT record negative reviews, so maybe you'll see 2 people raving about a product while 8 or 9 people didn't like it and just didn't want to record a negative review. Not to mention, some people are just raving about products because they're paid to or because they got them for free, so they feel obligated to give a positive review. (I honestly don't think that's THAT common, though.)
Really, I'm just the complete opposite of my sister: she's so impulsive it hurts, and I brood over things for days. I will literally fill carts with makeup, then get halfway through the purchasing process and just STOP because I'm not sure if the products are worth it. (Poor college student and what-not.
Having a bunch of reviews of the same product really does make me feel more confident in my purchase, versus only one or two reviews.
Also, I think having a slew of reviews on a certain product provides you with information one reviewer may have left out. Like, how well a certain pigment blends or whether or not a certain lip gloss bleeds like woah. It IS a little annoying when 10 reviews of the same product come out from 10 different people all at once, but I think it's because one person reviews it, others hear about it, and then they all rush out to try it at once.
Essentially, I'm saying I understand the annoyance, but I personally like having a ton of reviews, even if they can get a little repetitive. It makes me feel a bit more confident as a buyer.
Besides, if it weren't for extra reviews, I'd have probably sold my left leg for a few small pots of Lime Crime shadow when I can just get the same stuff at a fraction of the cost and double the amount from TKB Trading.
Now, obviously, nobody likes reviews that are just advertisements in disguise. Those tend to be annoying, fakey, and blatantly obvious, and I lot don't even show the product in action, so I try to avoid them.