Accessory work or bodybuilding is a training tool that is really under utilized in the CrossFit world. Over the years it has improved as people began to see some benefits like being more resilient to injury and their main lifts jumping up in weight.
The biggest flaw tho is everyone who goes to a CrossFit gym thinks they need to end up on the ground rolling out of breath at the end of every workout. This is not the case at all, having 1-2 pre-planned power intensity days each week can be very beneficial to all of you clients.
These low days will allow them to push the intensity harder on the high days, so they get more out of that training stimulus. Then on the low days they can really focus on performing body building work on muscle groups that may be lagging or underdeveloped. This will in time allow the body to operate at higher level of the weakest linked is improved. These low days are also a great time build up an aerobic base, which helps your recover faster from all training