The option is also there that whoever did kill the two individuals wanted them found for whatever reason - a warning, a message, whatever. And so the idea was to put them in the Avon which as another commenter mentioned is tidal and would likely wash them up onto the mud banks. I would imagine that with a high level of decomposition, with few if any DNA links to the killer after sitting in the water for so long would be a much more difficult case for LE to crack, especially when they're not connected to the city in which they were found, and are being investigated by a different police force. However I do think this is an unlikely theory.
As I said in a previous post before the London link came in, I also agree that the suspect being looked for is not from Bristol. Anyone who knows Bristol knows of much more secluded spots than the CSB, even in the immediate area. I think the likelihood of the guy being used as a mule on an 'ask no questions' basis is quite possible too.
To murder and dismember two people, pack them into cases, and transport them ~130 miles to a different major city isn't exactly an easy operation, and requires some level of planning, even if it's bad planning.
The Severn Bridge theory is interesting, but I think that's also a bit bizarre. For those that know the area, the Severn Bridge is literally a motorway (M48) that goes from Bristol into Wales. It's not a pedestrian friendly bridge like CSB. If they were trying to get to Wales, and got the wrong bridge, that is a possibility. But then why go to Bristol in the first place, when trains run regularly to Newport and Cardiff?
However you look at it, CSB is a really odd place to dispose of bodies. The pub is also another curveball. That pub is right by CSB. How/why did the suitcases and the individual get to the pub? Why did the taxi take them 2 minutes drive (if that) down the road and then drop them off? I also suspect the taxi driver stopped unexpectedly and kicked the man and the cases out, hence why he was seen panicking - the plan was all going wrong.