I have never heard of anyone that would cross another breed with an Arab to get "more" bone. Arabs have denser bone than other horses - they are the ones used in crosses to add bone density...
Medical evidence to the contrary
(but then people do many things based on what they've heard, but that aren't fact)
All things being equal (proper amount of CA and prober P to CA ratio, other minerals...while "mommy" is carrying it around) all horses are born with the same bone density. Additional bone density is the result of modeling and remodeling which comes as a result of what's done during the life of the horse after it's born (and bone size can be increased this way too). Arabians are born with no more bone density than other breeds.
Friesians are popularly used for their heavier bone, because they have a heavy build it results in heavier/larger bones. This allows you to have a head start on getting the size, but the process of modeling and remodeling for density is the same as with any horse....proper load bearing for stress is what increases density.
It's not the Arabians bones that make it so popular and successful in endurance racing or for cross breeding. There are plenty of horse breeds that, like the Arabian, have short backs, with wide (and wider) loins and can have their bones modeled in the same way for equal density. What makes the Arabian stand out for certain things is that as a breed (all things being equal) it produces animals that are faster then the other breeds which have short backs and wide loins. For example the Rocky Mountain Horse and Friesian (just to grab two...there are more) have short backs, wide loins and often (especially in the Friesians case) a more powerful build, but all things being equal (the same diet, conditioning, training, bone modeling, etc....) non of the other breeds can on average maintain the sustained speed that an Arabian is capable of. So while they can finish the endurance race still in good condition it's unlikely (virtually impossible) that they'll ever come in ahead of an Arabian that's been equally well conditioned.
The Arabian has been popular for cross breeding in the development of new breeds or for improving existing breeds for two traits.
1. Most often for endurance with speed to improve racing ability which is how the TB was created (they can out run an Arabian, but can't maintain it for as long)
2. In cases where the desire was to lighten and/or reduce the size of heavier riding breeds (although it's not as common a use today).
Just as Friesians were most often used:
1. To cool off hotter breeds giving them an easier attitude.
2. Provide more size in bone, muscle and strength.
Of course today a you find people using them to add a "look", but that wasn't the traditional reason for adding their bloodline. Outside of a show ring or movie industry I don't see having a "look" serving much of a functional purpose. (Sorry to all those show people who are into that, but that's just my opinion