Considering you can actually roast a target with single heat sinks to death, oh yes that 15-point heatup vs. the constant six is very, very useful.
I managed to catch a Rifleman by surprise in a 3025-era match with a Whitworth-S.
He hit me with both AC's and a large laser + walked. +1 overheat.
I pasted him with both SRM racks full of inferno rounds. Eight hit, +15 more heat, shuts down but otherwise completely undamaged.
I then stood there and napalmed his helpless machine repeatedly until the ammo cooked off, which actually took two turns (failed to restart, succeeded on ammo explosion roll turn 2, failed ammo roll turn 3 and *BOOM*). Not quite as satisfying as shoving a virgin Axman off a cliff to it's death, but the look on that guy's face was worth every second when he realized what I was doing. If he'd been smart enough to dump ammo I would have just added in the medium lasers + kick every turn until he stopped twitching, but the idea of someone baking his machine till it popped just made the idea leave his mind.
It won't stop an energy boat, but it'll sure as heck make it slow down greatly- even a single hot SRM-6 salvo is enough for that. People just don't use heat effects cause they're conditioned to believe that double sinks make it ineffective. Not at all. All your opponent has to do is push their sinks, which most intelligent opponents will do. If he drops firing weapons for fear of being flamed, you win either way- a heat crippled opponent, or less firepower incoming.