1) That's sandbox. 2) Percies are much slower (0.5 speed slower). 3) If you see 10 percies, would you send your ML right at them, while they're in formation? 4) Percies take much longer to build... even if I focus on eco, getting all my defendable mass extractors to t2 takes 10 minutes.
All theory. Speed makes a considerable difference. Put it into a real game. You see a ML being built mid in the water on setons. You are beach. You switch to making percivals, since that ML is likely to roll right through mid or possibly right up on your shore, and air superiority "was" even, as your air player is now switching to torp bombers or gunships or mercies, and is now behind. If that ML avoids your percies on beach, or goes up middle, i bet your percies will not be very effective. If it goes up middle, your percies will likely be strung out, and you will need to invest time in grouping them up, so they all shoot at once, instead of getting lasered down 1 at a time. On top of that, what if your omni was mid and you can't see it? Percies not so good there?
What if your back guy made a GC instead and you made T3 PD and the ML went mid? If the guy controlled the ML saw the GC, with 2.5 speed, the ML would be able to get away with 2.7, especially if the GC stopped. With 0.2 extra speed, it just has enough to create a gap and create a line of destruction on the run through a side base, though damaged. It wouldn't be able to stop and target your T3 PD, but it may stop to target your ACU. What if they had no omni, and had to rely on scouts? The speed gets targets in laser range sooner, similar to extending the range, but without creating out ranging balance issues. Sound useful for such a very slight change? Drastic changes suck.