On the Mac (since Lion) the scrollbars are invisible, by default, until you actively scroll. Some web applications, notably Office 365 and Gmail, create their own scrollbars that are consistent across operating systems and are always visible. In my own experience I've noticed users, including experienced users, not realize that there is a block in a web app that is scrollable because the scrollbar is not visible. This seems unacceptable. Is there any reason not to force scrollbar visibility by creating a custom scrollbar as Gmail and others have done?
Google uses custom scrollbars for a number of reasons, but one of them is cross-browser consistency. Wen building a web application like Gmail, creating your own scrollbars solves a number of layout problems since every platform has scrollbars of different width.
If you build an application for OSX, I would advise against creating scrollbars. They are hidden because you don't interact with them if you use the trackpad. If you connect a mouse and use that, scrollbars will appear because you can use them. Scrollbars should never be your sole indicator that you can scroll towards more content.
Users of a Mac will be dealing with scrollable content the entire time they're using the Mac, in each and every application they use. They will have learned how to deal with it. Having scrollbars in your application will not help them use the other applications they will use throughout the day. If anything it will create confusion about when or why to expect scrollbars. For system-wide features stick to the system guidelines and defaults if at all possible.