As Jazzik points out, by adding the word "and" Honda effectively say ' use B if you want to.'
But i ask myself "why wouldnt you want an increase in regenerative braking ? ' If its beneficial most of the time why not make B the 'normal' and D an option only used if you want ACC. It could even be automatically switched to D mode operation when you select ACC.
And why does it need D option at all? My guess is that the ACC would struggles to maintain and reduce speeds using regenerative braking, and a constant shift between higher than normal regenerative braking, and the occasional need to apply mechanical brakes would confuse the system and the driver. And could well have an adverse affect on fuel consumption.
ACC generally uses less fuel because you are maintaining a steady speed, but when it needs to reduce speed then increase it again it may do so quite aggressively which uses more fuel . B mode level of retardation may make this worse. And may do so under similar road conditions (main road as well as motorway,) even when controlling your speed manually without using ACC.
At the risk of repeating myself, my attitude to manual brakes is 'use them or lose them. ' I have twice needed to replace rusty disc rotors,and often needed to overhaul gummed up moving parts on cars that only got occasional use. Its a long time since I ever needed just to replace worn disc pads . If you primarily use regenerative braking in B mode, your brakes effectively get little use, even if you use the car regularly.
Coming down a longish hill in B mode the HV battery may be full long before you reach the bottom. You cannot add more charge to the battery so there is no longer any free energy and the car needs to find other way of dumping the excess energy and retarding the car as best it can. I believe it sometimes utilises genuine engine braking using compression of the ICE engine , but this can be quite noisy. I dont suppose this ,or putting more power into the electromagnets when in B mode , adds any significant strain on the drive chain , and it happens to some extent even in D mode , but I'd rather just use brakes more. Replacing brake pads if necessary is no big deal expense wise. .
But ultimately its whatever works best for you, and may depend on where you habitually drive, particularly in town.