1. What are the total possible points out there? With colonies (available in what 15% of games), VP chips, and Alt-VP, the province pile is not the be all, end all of the game. I would hazard a guess that in the majority of random games, you have some way to beat a 5-3 split beyond just 3-piling.
2. Engines, much more than Big money, can destroy/steal enemy points. Sure if we both go for engines, adding in some Sabs isn't that great. But if you follow stock big money, the first 4 or 5 provinces come quick while the last few come really slowly. This gives me a lot of time to take "poor" attacks like Sab, Rogue, or Knights and kill off BM's golds & duchies, if not provinces. If I end up engine vs engine, it might never get to the point where adding in the attack and stalling the game while I wrack up duchies is viable. The mere possibility that one person can get thwacked repetitively should change strategies.
3. Engines should be built up to their maximum or until the other guy starts going for green. Take something like Nv/Bridge. It isn't a proper engine, but if you end up with a 5/5 split on both you get $12 coin with 5 cost reduction, this in turn gives you 4 provinces and 2 duchies as your max payout. That might handily beat BM. But against another Bridge/Nv guy, particularly if he can toss in some cheap +buy like Pawn, Market Square, Candlestick maker, etc., it can still mean a loss. For extremely high yield engines, you have to make different tradeoffs so that either you get enough points in one turn to lock out your opponent or if you do get points - be sure your opponent can't build past you for the win. And this does have a big effect with some cards. City is an obvious trap - you want to trigger your big pile of Cities, but not your opponents' (as he will then pile something else out and buy an estate for the win).
4. Engines allow a lot more flexibility for dealing with what the other guy does. Take something like Envoy vs Spice merchant. If you go Envoy early, you are pretty much committed to BM (and Envoy is a strong BM enabler). But what if you have a setup where BM > engine > slog > BM (BM can get 50% + 1 of the points before the engine, the engine can block the slog by buying Silk Roads and turning them into engine components via Upgrade, but the slog can build up monster Silk Roads that trounce the Provinces by the time BM gets all the provinces). You want to delay your decision for as long as possible. So you open Spice merchant. You thin out some copper, if your opponent drops into BM, you go slog. Your opponent knows this, they don't go BM. So they buy cards that don't work with BM, you then either drop into BM (if you still can) or into engine (as it beats slog). If you need to make a quick transition to mass Silk roads, engine isn't that bad. Yeah terminal draw can hurt, but most other things +buy, +actions, etc. don't actively hurt. If you have TfB, you might even cannibalize your problem components into slog cards. All of this delay just makes the real race start later - you spend more time doing suboptimal things, but things that don't lock you into a losing choice.
Because at the end of the day, you only care about the relative score. If you aren't facing BM, then don't play against it. If your opponent starts building a deck to win the race to 4 provinces when you can handle mass duchies better, you've just won.
