Exactly what Frosst said. Stag is actually CMT's in house brand. CMT has been in the business for over 40 years and supplies LE and Military. They actually are a DOD Certified Subcontractor. They manufacture parts (or have at one time) for some of the top brands like Noveske, Colt and Wilson Tactical etc as well. Their fit and finish are second to none. Spikes are nice as well and I believe they are supplied to them by Aero but not exactly sure. Like Frosst said it's personal preference and experience with them really. Most lowers and parts are made by the same manufacture these days with many coming off the same assembly line. There are many great Mil-Spec lowers and parts out there. I would say from what I have seen that LMT, CMT, and LAR make some of the nicer lowers though in terms of fit, finish and quality.
As for me I have been using a stag lower and upper, LPK, BCG and buffer tube for a while now with no issues. The fit and finish is great and those are the reasons I am looking for another. I am sure a spikes along with a few others are great but I have no experience with them personally.
As for why I don't buy the complete lower.. I already have a Geissele SSA trigger, LPK, Magpul grip and stock, Stag 6-position heavy buffer tube. Need a lower and few other things to complete my build.