The "extended warranty" that is sold is one of the highest profit items for a retailer.
Same goes for the "equipment protection plans" or whatever they call them these days.
Figure what the chance of you using the plan/insurance is over the couple of years your phone is worth replacing. Remember that they will probably replace it with a "refurb" and not a new phone. Multiply the chance of you using it by about $200. (A new Nexus is $400, will probably be worth $200 in a year or two, and $100 not long after that.) That $8 a month is about $100 a year. In two years, you could have held onto the cash and, if something happened to your phone, easily bought a replacement yourself, or used the $200 to trade up to your next shiny object.
As pointed out above, even if you do buy it, what does it really cover?