Well, ANY keyboard with a sampler should IN THEORY be able to do synced audio loops, but the difference is, take a look at any WS that does this... It has a whole graphical section to do exactly what you say you need - make it simple, make it easy! Plus pads to put these loops on, to allow you to 'play' them. They also have pitch and speed independence made simple, and then contemporary loop manipulation tools like slicers and 'stutterers', easy filtering for the loops, you name it.

No matter HOW you slice it ( ), modern arrangers don't have a fraction of the tools that modern WS's do...

Perhaps Roland's SEEM easy to use because they don't have some of these modern tools (no sampler, for instance), but I have to agree that some of the things they DO have in common with other arrangers, they somehow do manage to make them much more intuitive and easy to do...