The quality of the photo really doesn't let us know whats going on. If you have an inspection lined up tell the inspector your concerns about the streaking. You can also ask for a roof inspection from a roofing company in addition to the home inspection. Most (not all) roofing companies give free estimates. If you are not going to replace the roof offer to pay the company for an opinion of the serviceability of the current system.
First thing to remember is that E.P.D.M. (rubber) roof systems are NOT warrantied on residential applications.
There are flat roof specific systems that are warrantied for residences.
Any roof system will at some point in time require maintenance. The average life span on a single ply roof (rubber) is about 15 years. The biggest thing about maximizing the life span of any roof system is proper installation. This is the single biggest reason of premature failure in the roofing business.
Good luck and hope all goes well.