Wait, is this related to old side-new side?
This really belongs in the original thread about RZ, not here. But the Old School/New School debate was in the 1800s over the Second Great Awakening under Finney and his fellow-travelers. The Old Side/New Side debate was in the 1700s over the First Great Awakening. Two different but related issues. It's possible to be New Side but Old School, i.e., a supporter of Edwards but a fiery opponent of Finney, and there are probably plenty of people on the Puritan Board who would, if pressed, describe themselves that way.
There were other factors in both movements and that's where things get messy. It is entirely possible that RZ does not fully understand the group to which he has hitched his wagon when he supports Nevin and Mercersburg.
The Mercersburg Theology was an attempt, primarily in German Reformed circles but spreading beyond them, to re-emphasize a Reformed view of confessions, of catechesis, and of the importance of sacraments and liturgy, in reaction against the Second Great Awakening. Most of that sounds good, until we get into the details. Most people who supported the Mercersburg Theology not only opposed the Arminian revivalism of the Second Great Awakening, but also the emphasis on personal conversion that was the key impetus of the First Great Awakening. Exceptions exist because history is complicated, and the allegations that the Mercersburg people were "dead orthodox" and "ritualists on a road to Rome," while they may well have been accurate for some (especially some of the leaders) can't be fairly applied to everyone in that movement. There were people in the Mercersburg circles who were sincerely converted but were horrified by the Finneyite craziness, and reacted strongly by returning to an emphasis on creeds, confessions, catechisms and solemnity in worship, i.e., recovering the historic Reformed position.
RZ wants to be an "ecclesial Calvinist" with an emphasis on being part of the historic Christian church rather than lapsing into "sectarianism" and "me and my Bible" broad evangelicalism. Paying attention to the importance of the institutional church, and doing theology in community, is a major part of what Mercersburg was about.
I think RZ likes that part of the Mercersburg tradition and may not understand its later development and what it opened the door to bringing into America out of the very worst parts of German theology of the mid-to-late 1800s.