I think most people see a strong connection between (i) performing an ordinance and (ii) serving as a formal witness of that ordinance. The two actions are not the same thing, obviously, but they are intertwined. In my view, both are beneficial in allowing people to build ties through shared spiritual experiences. That's why parents I know in the church want their sons and husbands to perform ordinances (yes, children do perform the ordinances of sacrament and baptism). Doing so gives them a spiritual experience that will bring them closer to Christ and help keep them rooted in the gospel. Likewise, I (and I imagine other parents) will now want their daughters to serve as witnesses. Doing so gives them something they would not otherwise have. Otherwise, why open this door and encourage them to participate?
So I'm still puzzled as to why members would be encouraged that girls and women can now have more involvement in ordinances (by serving as witnesses), but would not also desire them to have a more complete involvement (by officiating). The common tie is that both actions are good. We want all people to have access to all good.
As for arguments from silence, my reading of the scriptures suggests that God stays silent until we actively desire something and ask for it. That's how Joseph got the first vision, Emma got the WOW, the primary and YMMA were formed, temples are built, and on and on. If God wants women to be ordained, He's not going to direct that until we as a people actively desire it and ask. Sitting by passively will not cut it. We have to be actively engaged and wrestle with whether this is really a good thing.