Well, the translation of "concubine" for mekake, or sokushitsu, is not ideal.

The character used for a secondary wife, 妾, has a somewhat different connotation in Japan:

Especially in medieval (and early modern) Japan's society, it refers to any wife except for the principal wife (which we refer to as tsuma, or later, seishitsu).

This means, these women were actually legal wives, which is not the case with concubines. The main difference is that the role of representing the household vis a vis third parties in the case of the husband's absence, illness, or premature demise, was the duty of the principal wife, who therefore had a dominant position. Suffice it to say, this could become quite troublesome in the case when the designated heir to the household was not the son of the principal, but a secondary wife, even moreso when both had sons!

Now, marriage is also one of the primary means employed by warriors to create alliance networks, and thus rarely considered a losing situation for the participants; marriage also was understood as a social contract between two ie (families, or houses), not between the individual people involved.

(I have lost a few words on the institution itself in medieval Japan a few weeks earlier here, which might be interesting)

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Either way, would you happen to have a few examples that you refer to here? Then I could see from which families these women actually came; the Sengoku daimyo I took the liberty to take a look at were, for the most part, only married to a single woman (which was, apart from the Ashikaga shoguns, still considered the norm in warrior society of the time), and even if not, all their wives were women from the warrior class.