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It means they'll generally miss (or get stuck in) recirculation zones. It also means if you release from the inlet and have (for example) 20 that there's a very good chance they'll not be very representative of a complex flow. There's a reason we may use pathlines, vectors and contours, and not everything every time. 

CFD Post is uses an interpolated data set from Fluent - it's covered somewhere in the manuals. For a well resolved flowfield (ie lots of mesh where the flow gets interesting) the difference is negligable. Fluent's data is exactly what the solver calculates so I always use that, even when I did (rarely) use CFD Post.