Soap does not replace a legendary doctor. Or even a novice. Doctors > Soap by far.
Well, the soap is applied by dabbling doctors, I don't expect the soap to do the work
all by itself
. My experience with building nice hospitals, with a dedicated cistern, full supplies and doctors on call 24/7 (at least 3 doctors to prevent on-break breakdowns, bedrooms next to the hospital, possibly burrowed) is that injured dwarves quickly get up and walking again, even if treated by dabblers. I deliberately injure lots of dwarves, and I'm darn impressed how quickly and reliably my medical trainees have them back in the Very Dangerous Room (in fact I'm so impressed, I'll probably cease my doctor-training programs, your garden variety peasant seems to be just as effective a doctor as he would be a mason or cook)
With this in mind, investing points in doctors becomes extremely dubious. I mean yes, getting some immigrant bone doctor is nice and may as well put her to work, but why spend points on medical skills if it doesn't improve survival rates, which AFAICT it doesn't at all, and since recovery is fast even with dabbling doctors?
The other thing, is that "prevention is better than cure". At the moment all injuries are player-caused or highly preventable except those to military dwarves. There are no accidents, a mason wont crush his hand, a miner wont put his pick through his foot, a wood-cutter wont get hit by a widow-maker. And cave-in injuries are 100% preventable.
So when I figured out, the only reason I had a doctor was to treat my elite military, I figured it'd be smarter to have a military which is better at not getting injured. Prevention is after all, better than cure. So I replaced the starting doctor with a starting military dwarf with skills in Shield User, Armor User or Dodger, and a weapon skill since "the best defense is a good offense" (the quicker you kill the enemy, the less injury they can inflict).
So even if there's something I'm missing about medical skill, such as masterwork bones that are 2x superior to the original, I think that embarking with military skill (and putting it to work ASAP) is just plain smarter than embarking with medical skill.