BS”D
These “lo matim” (profil 24 which gives no reason for rejection – actually the precise term is ee-hatama – as opposed to 21 which is mentally unfit and often obtained illegitimately) guys whom I know and see every day are very successful Chabad shluchim abroad. They are healthy, strong men in every sense.
They are lo matim precisely because they are now shluchim; the army was afraid they would influence others toward Yiddishkeit as they now do outside E”Y.
If the army allowed for proper standards of Yiddishkeit (and Nachal Charedi may be the answer for some) then the army would be an option. But the army, besides being an army, is meant to be a social equalizer and indoctrinate certain values. As such, no charedim can serve unless they are very well grounded in Torah from previous intensive study
On the other hand, charedi organizations (Zaka, Ezer MiTzion, Yad Sarah, Ezra Lamarpeh, Meir Panim, Kolel Chabad etc etc), shoulder much of the social service burden after the incompetent government does the bare minimum for the ill, poor and disabled. It is only proper that service in such an organization is counted as national service.
Charedi girls marry early and national service is not an issue. Many Chabad girls, who do not marry quite as early, in fact travel abroad for a year of community service before marriage; they do a lot more than those who fill the often make-work desk jobs in the army and Sherut Leumi (though the latter is still more productive and morally safer than the army for girls).