I'm not sure how you are wiring everything but Amps can draw as much as 50mA each and many spike that level if the main fuse in the amp blows. I am told one brand of amp spikes as high as 500mA. It will also protect your CANBUS adapter in case of a short circuit. I'm not sure what the trigger current is for the 2140 but to me, a $5 relay is good insurance money, that's all.
Also you have a delayed 12v out for amps from the 2140 plus you'll need clean 5V supply from the 2140 for the hubs so I'm not sure that you need to trigger more than just the 2140 with it but that depends on how your wiring everything. The PAC RP4-VW11 (for example) gives you 1 amp to work with on the acc. output but regardless, IMO, a $5 relay is not still a bad idea to protect the adapter.