Here's a quick copy/pasta from a blog article I wrote awhile back. Maybe this will help shed some light on reasons to either use parallel or serial loop configs.
Parallel and Serial Loops
The main concept that is often misconstrued is that loop order really isn’t what gives the differences in performance, it’s the flow rate. When you are considering the differences between parallel and serial configurations, you need to examine the differences in resistance each configuration has in your loop. Every block and radiator in your loop adds resistance to the flow of the liquid, therefore the more resistance, the slower the coolant flow.The ideal coolant flow rate in a liquid cooling loop, regardless of components and configuration, is 1gpm (Gallon/Minute) or more. At 1 gpm it takes 250 watts of heat to raise the water in your loop 1C. DDC and D5 pumps can achieve 1 gpm of flow through most standard loops, such as a CPU, GPU and two radiators. Which is why you often see serial loops in a lot of standard water loops. It’s only when you get to more complex loops like SLI and Crossfire GPU configurations that you see parallel and dual loops being used. A parallel GPU configuration alone, is helpful because the resistance in the loop dramatically decreases when the blocks are put in parallel instead of series. The reason behind this is because when you put resistive components into your loop in series, the resistance is additive, however when they are in parallel, the total resistance is less than the smallest amount of resistance in any one of the components. The one key to having a parallel GPU configuration is making sure each GPU block is identical in resistance, or else the flow rates between each block will vary which will result in inconsistent cooling performance. This can be avoided by simply purchasing identical blocks for your cards. If you plan on putting your CPU in parallel with the other blocks in your loop you will want to do some research, or just plain old testing, to see if the difference in resistance to the other blocks is too great to achieve acceptable performance.