As much as you are correct in your assessment, there are times where it's beneficial to have a very slight negative pressure (when performance is maxing out).
Thankfully, none of the fans are static. When your system isn't being heavily used you can actually zero fan the H115i and you will have a positive pressure system (2 fans pushing against 1 pulling). This is excellent for repelling dust etc. When performance is needed you will potentially have a negative pressure system (It will definitely be negative once the H115i is on song, that's for sure) but this isn't a bad thing as you will have fast moving air and plenty of it without the noise normally associated with such flow.
Thankfully, none of the fans are static. When your system isn't being heavily used you can actually zero fan the H115i and you will have a positive pressure system (2 fans pushing against 1 pulling). This is excellent for repelling dust etc. When performance is needed you will potentially have a negative pressure system (It will definitely be negative once the H115i is on song, that's for sure) but this isn't a bad thing as you will have fast moving air and plenty of it without the noise normally associated with such flow.