As far as "pure" base training goes, my experience is that even on z2 rides I need to punch it up on short hills here and there at least into z3 for short periods. If I were using a PM, I'm probably actually into z4 here and there. Plus all winter I do one competitive group ride/week. In general I keep it down a bit, but do try to get in about 1 hour of z4 on that ride, including some sweet spot, a little LT, and even a tiny bit of z5. If I don't do these things, my performance goes to the devil, including my z2 cruise speed (watts). I don't think that's a good thing. Looking at it from a PM perspective, I guess when my FTP falls off because of not doing hard work, then my other zones drop off too and my legs don't get the work they would get if I were looking to put out more watts during base training. Does that make sense? I think this is borne out in the literature.