Keeping in mind that the majority of Southern Baptist churches are not Reformed, the question needs to be asked: Why does a Reformed Baptist remain in the SBC?
I certainly cannot speak for all Reformed Baptists within the SBC, but I'll give you my answer. The sole reason that I remain in the convention is international missions. Frankly, I see this as the only reason for the current existence of the convention at all.
Having served with the International Mission Board (IMB) in South Asia, I can say that I am still impressed with Southern Baptist work overseas. Both Alice and I are pleased that the focus of the IMB is church planting. This seems to be the only strategy that will be effective in reaching the thousands of unreached people groups with the gospel. Church planting relies on little outside support, depends upon national pastors, and allows the gospel to be understood within any cultural context. Although we are no longer with the IMB, we will continue to support it financially and through prayer.
Within the SBC, most of the local churches are small. We are a convention of small churches. Most of these churches cannot afford the expense involved in sending even one person overseas to serve as a missionary. This also does not take into account the training the person will need prior to going. The local church can, however, give to the SBC's Cooperative Program (CP). Fifty percent of CP income goes straight to the IMB; the remainder is split up among other SBC entities.
The money from the Cooperative Program, combined with the annual Lottie Moon Christmas Offering, supports the IMB. This money allows the IMB to adequately train missionaries to live overseas, adapt to new cultures, and work with nationals to plant indigenous churches.
I'm sure that other evangelical denominations/associations/conventions/sending agencies do a fine job with international missions. I praise the Lord for their efforts and hope our IMB people will work with them. The reason I support the IMB in particular is that I have seen IMB folks in action and I remain impressed.
As for many of the other large SBC entities (such as NAMB, ERLC, Lifeway), I think the local church can do almost all of what those large and expensive agencies do. We do need to ask within the SBC why we are spending so much money in areas where the local church should be taking the lead. Alas, that discussion remains for another post.
So the answer for me is clear. There is one, and only one, solid reason for a Reformed Baptist to remain in the SBC. That answer is a large and important one - international missions.