You guys always alert me to the really obscure (and for me often most interesting) hadiths!
If you want a mystical explanation, I can give one, although of course I don't expect you to find it plausible.
I have some cryptic notes on Safiyya here:
http://thegoodgarment.wordpress.com/2009/05/04/safiyya/The key to my understanding of who she represents is the hadiths in which Muhammed says "your father is Aaron, your uncle is Moses and your husband is the Prophet". This configuration is EXACTLY how Kabbalah describes the Shekhinah, the feminine presence of God (and, as I have mentioned in previous debates here, within our own psyches, the female, creative anima within ourselves, whether male or female). In all the standard Kabbalic cosmologies, Shekhina is mythopoetically related to be as a daughter of Hod (the receptive nature of God, equated at the human level with Aaron) and Netzach (the providing nature of God, equated at the human level with Moses). Shekhinah must me married with Beauty, Tiferet, the light of Prophecy in order for balance to be achieved in the human soul. Her other name is the Sabbath bride and this cosmic marriage is what orthodox Jews celebrate every Saturday.
Shekhina is Sakina in Arabic, and both the Quran and Kabbalic sources all understand that this Sakina/Shekhina is what was present (again, mythopoetically) within the ark of the covenant.
There are a couple of other similar things going on in other hadiths which narrate how Muhammed bought her (for 7 camels, for instance -- again, Shekhina is often said to be equal -- or to "contain" -- the 7 names of God).
But this Hod-Aaron/Netzach-Moses/Tiferet-Prophecy relationship, for me, intimately binds the Kabbalic understanding with the hadiths surrounding Safiyya.
The prophet "stole" Safiyya in these hadiths. But, more importantly, what else is being "stolen"? The Kabbalah itself! And more than that, Shekhina itself.
But the prophet then marries this Safiyya/Shekhina. The theft is transformed into the Cosmic Marriage of orthodox Judaism. So, basically, it's all cool.
Similar stories of theft of wives occur throughout Torah, and these hadiths are a repetition of those archetypes (see, for example, Bethsheba and David which on the outside looks to be in complete transgression of Jewish law).
Regarding then the hadith that he didn't sleep with Safiyya in rotation -- b/c the other wives were slept with, as has been discussed often, in sequence. Why? Because, conceptually, Prophecy was in union with Shekhinah constantly. So the reality is that the Prophet never stopped sleeping with Safiyya: their sex went on and on forever. They are still at it right now in fact, but this truth has been lost to us.
(Interestingly, my interpretation was recently put to a group of "fundamentalist" Sufis, who ended up getting pretty upset, mainly due to my understanding of theft and sex, which are very loaded words for them. They also tend to be quite anti-hadith, whereas I am hyper-Salafi. But interestingly, the use of Kabbalah distresses some, because the mystical version of Islam, just like the mainstream Islam, appears to have got quite segregated with respect to its piece of the pie. However, whoever wrote that hadith was, for me, clearly well-versed in the Jewish mysticism: the language is simply isomorphic. Having both Jewish and Islamic backgrounds from my mother and father respectively, you could argue I am simply viewing everything with my peculiar personal perspective, but on the other hand, I suspect many of these hadith writers had a similar background to mine!)
(Hot and sweaty) Love and Light,
The Tailor