not sure if you noticed but standard burn is way more manouverable than sustained burn
And when is that a useful advantage? A target fleet that's trying to avoid contact is pretty much exactly as maneuverable as you are when both of you are at the same burn level, so you either want to get on them so fast that they can't dodge in time - which means coming in hot on either Sustained or Emergency Burn - or you want to be sneaky enough that you're on top of them before they know you're there - which means running dark until you're ready to pounce, unless maybe you're running an extremely stealthy fleet. Standard burn isn't useful in either situation except maybe to close that final little distance when you sneak up on someone and jump them. If I'm the one trying to avoid being jumped, I either want to be stealthy or fast; they're not going to come after me if they don't know I'm there and speed - especially speed at least equal to potential interceptors - is a very strong defense against interception. Maneuverability, meanwhile, is heavily reliant on timing - duck too early and they'll just make a slight adjustment, dodge too late and they'll be on top of you before you're out of the way - and can have serious reaction time issues when a hyperstorm throws a hostile fleet at you from out of the blue while you're running at accelerated time because flying through hyperspace is, by and large, boring.
Also, I find that if I need to make sharp(ish) maneuvers, pressing and holding 's' while at Sustained Burn is usually good enough, and while it doesn't have quite as much acceleration the higher top speed is usually more valuable, because the probable reason for me needing to make a tight turn is that my target bolted. I'm probably not going to catch up with them until their Emergency Burn wears off anyways, but if I'm barreling after them at a Sustained Burn level of 18 or 20 and paying attention then I'll probably be on them before they can get out of the way when their Emergency Burn wears off. Beyond that, if I really need maneuverability, Emergency Burn combines the speed of Sustained Burn with the agility of standard burn, so it's a much,
much better 'pounce' or 'dodge' than standard burn as long as I'm willing to pay the costs for using it.