nosql, who needs it who doesn't?

i'm seeing a lot of start up using nosql instead of relational database.
why is that?

90% of them don't even take off to have more than 100 users.
robust mysql or postgresql trumps finicky nosql bullshit anyday. especially if you don't need nosql at all.

is nosql just a fad?
or is there actual improvement of using nosql in some cases?