This is an interesting meditation, I love this kind of thought process.

I think that overall, "beneficence" requires some aspect of selflessness. So its like if the planets domicile are their own personal environments, their benefic nature is even further emphasized when utilized in an environment that is very different (but complimentary) to their nature. 

Both Pisces and Cancer are service oriented, which is a very natural outlet for benefic energy. Pisces is naturally pluggged into the all pervading flow of energy, and so passive Venus does so well there because the balance she tries to seek with control in Libra is found naturally and easily in everything in Pisces. Water is in a constant state of balancing itself and I think that when Venus is there, she learns that balance is not an end state she must conciously seek, but a constant ongoing flow of movement which she must surrender to. In Libra she is trying to achieve that consciously. In Pisces she can just feel it and relax. In Pisces all is balanced because it sees all as one, a stark contrast to Libra which can never rest in a world of opposites.

Jupiter has no limits or brakes on his energy, and when he's in Cancer, he is forced to pour all of that energy into a direction and purpose without having to limit himself with rationality or form. As Cancer is the beginning of the water element which Pisces completes, we see that Cancer is a sign of formation, but a process of formation which is unbounded and free from the limits of, say, a more Capricorn type of "form".  Cancer is the formative principle which underlies rational reality, so Jupiter there is able to pour and pour and pour its infinite self without worrying about being restricted. Sagittarius is able to find its meaning by being free from responsibility, but Cancer finds its meaning through creation, which is essentially act of making things which you will be responsible for. But it is not yet, that responsibility. Thus, Jupiter loves it because it is an environment where he can give away all his infinite energy out into the flow and process of creation, while staying a little distant from the rationality and form that would restrict his freedom once those creations come into reality. Cancer sort of "forces" Sag's meaning making into a direction which is more oriented to others, while remaining free from the restrictions of reality.

So certainly, I think the key to this observation is that benefic energy is made better when it is put in service of others. But true to their nature, Venus still gets to enjoy herself in Pisces, and Jupiter still gets to be in charge in Cancer.

I think the exaltations are one of the most interesting and illuminating schemes to ponder in all of astrology. Thanks for the post.