I found it perplexing that the overall attitudes towards the BG were negative, yet they found themselves in the highest echelons of power through marriage.
There are several factors in why Bene Gesserit would make perfect concubine/wife. There are also serious drawbacks to having one, as well... So it's not something that's as easy as answer suggests.
- Bene Gesserit guarantees fertility
- Bene Gesserit guarantees perfect health of both mother and child
- Bene Gesserit guarantees acceptable lineage (to say the least).
- At the same time they are not considered House alliance material (at least not usually), allowing for political marriage to still occur.
- Superior training, however common knowledge falls far short of reality. But in this case think etiquette, sex, maybe economics or other household-desirables.
- As noted in comments - huge prestige boost.
- Speculation: access to some of the B.G. resources
- Loyalty to the order
- Ulterior motives of the order
- Unknown lineage
- Political implications (sometimes severely limiting political options)
The books mention a specific breeding plan (however, no idea if its existence is known outside B.G.), which often goes contrary to given House's ambitions and/or expectations. And definitely does not imply if the Emperor's deal with B.G. came with known caveat of only daughters, for example, or for Leto Atreides, for that matter.
The Bene Gesserit were nobles themselves. They were almost all descendants of one or other ancient noble family. In the Encylopedia of Dune, the family branches are traced and documented and many of the marriages were to early BG. History tells us that early abbeys were sanctuaries for many young women from noble families. Herbert takes this further by attributing special abilities to an Order that had endured for so long, and albeit remotely and veiled, they were enigmatic and curiously powerful. To ally ones family with the BG brought a level of respectability and an admirable practicality in one's governance. No matter the mystery that were the BG, they were suitable mates to even the highest Royals.
The ruling aristocracy also chose them because of the education that BG would impart. In Children of Dune, Farad'n nearly jumps at the chance to have a BG teacher, Jessica, even at the risk of losing his life if Ghanima was to have her way.
They were ultimately teachers, and finding a decent education for a scion of a noble house, and mate, well, they were the only game in town, so to speak. The BG singularly made themselves irresistible.