Basically this. The answer to the question "when do spring stats start to matter" is, to me, pretty much never. You mentioned that some pitchers will do things like throw only fastballs the first game of the year. If he doesn't get knocked around, will we fall back on our "pitchers are ahead of hitters at this point" cliche? If he gets knocked around, is that indicative of what he'll do during the season? And when we count up his stats at the end of spring, his ERA will include his experimental and rusty early starts, so what is a spring ERA worth? Then if you want to remove the early starts to make ERA more closely aligned with a real-game experience, you're taking an already small sample and making it tiny, so again, what is a spring ERA worth? This is on the well-trained staff of scouts, coaches, and manager. We can argue about spring stats until we're blue in the face (and we probably should, because otherwise it'd be boring around here), but not a single one of us could reasonably make a stats-based argument that would be better than the subjective judgment of the staff.