Ubuntu's unity interface has invited a lot of criticism since it's beginning. Unity's scroll bar, which is not visible easily, is one of the reason for inviting a lot of criticism.
Finally, after 4 years since introduction of Unity, Ubuntu has decided to change it's design decision. Unity will introduce GNOME like scroll bar starting from Unity 15.10 on wards.
According to a news article published in softpedia:
The famous "almost invisible and difficult to make appear when you need them most" scrollbars in Unity 7 are going away, and they are being replaced by the upstream version from GNOME.
The idea was to have scrollbars that don't take a lot of screen space, and that only show up when they are needed. It's a good idea, but they never worked perfectly. It happens all too often to hover the mouse over some windows just to find the position of the scrollbar. Yes, they are nice, and you can't find anything like it anywhere, but they are going away, and it seems that it's all for the better.