And since it's an omnimover, it never stops moving. So in theory, the line should always be moving, unless there is some kind of pre-show that they plan to corral people into in groups like at HM.
The line at the California Adventure version will never stop moving because they aren't doing Fastpass there. (Disneyland management is not as big of a fan of Fastpass as WDW management is. Very few Fastpass rides left at Disneyland, and when new stuff gets built like Toy Story Midway Mania or Little Mermaid, they don't include Fastpass with it.)
The Standby line at the Magic Kingdom version will stop moving repeatedly because they are offering Fastpass there. You'll need to get a Fastpass at the Magic Kingdom version of Little Mermaid.
That irks me. If the line is kept moving, then there is no need at all for a Fastpass in the first place. That's the reason that PPF is so ridiculously long despite the fact that the ride vehicle never cease moving.
An omni mover means that a new vehicle is dispatched ever 2-3 seconds or so. The chain of vehicles is steady (save the occasional interruption for a wheelchair or other issue). Having said that, it does not guarantee that line will always be moving. Sure, if people all walked at the same speed it took to load everyone in the vehicles, then yes the line would be continuously moving, but that's simply not the case.
Add in Fastpass, and there is a merge point for the standby and fastpass queues. Both Fastpass and Standby can be stopped at this merge point. Once guests get past this merge point, they would enter this area where the line could theoretically be continuously moving (but again, not really).
The benefit of an omnimover is it's large capacity, this is a function of the fact that it is continously moving. However there are attractions that are not omnimovers, but have a larger capacity than omnimovers (Kilimanajaro Safaris is a great example). The need for Fastpass is predicated more on demand vs. capacity as opposed to the ride system. I expect that the newest attraction (Little Mermaid) in the most popular theme park in the world (WDW's Magic Kingdom) will generate a higher demand than the same attraction (Little Mermaid) in a park that has 25-30% of the attendance (DCA).
The capacity of the attraction in DCA will be the same as it is in MK, but the demand will be substantially higher in the MK. As such, the MK version should require Fastpass moreso than the DCA counterpart.
As for Disneyland's preference towards not using Fastpass, I think it is a function of space (queues with both a Fastpass and Standby queue take up more space than a standard Standby queue) as well as necessity. Disneyland certainly uses Fastpass on attractions where it is necessary, and I think the best approach is to have some attractions as seasonal Fastpass attractions (like Haunted Mansion in Disneyland).
