I'd still say it's pointless vs bosses; if you're getting into melee range, you might aswell start backstabbing, which is the optimal damage option.in CoF (I think 76 in explorable) I hit 7,5k backstabs near-consistently thanks to 100% crit chance. Just dodge faster and use your stun breaks before something bad happens. The faster they die, the less time you have to spend worrying about getting dead.
I reference in my post soloing certain vets and champs, not bosses in dungeons. Soloing a champ mob with dagger spec is likely not possible as most of those champs or hidden champs (certain vet mobs) will 1 shot any thief if they land nearly any hit or without any other target you will be forced to tank them and even thief endurance can't go long enough to solo a champ by melee.
Regardless if we want to talk about bosses in dungeons: In FotM for example on the 1st boss dredge fractal, SB works quite nicely as he summons 2-3 adds every so often, 1 of which being an overly tough vet. The last boss for example, SB is also a good way to port around to reach the smelting pots while still doing some damage when he has the debuff on him. You can backstab that guy all you want if he doesnt have the debuff on and it does nothing. Likewise his AE lasts longer than a dodge and the range is extreme, additionally he throws out 1 shotting mines after. Staying out of range of the mines is a workable tactic in additon to the need to kite him to the smelting pots (found best done by ranger as thief loses aggro too easily) and getting someone up there quick is fastest way to get him dead (thief moves faster than any other class).
There are also several bosses where melee is unadvisable or you will be dead as they have stages where they AE either every so often or near continuously. Regardless of how big your backstab spike damage is, or death blossom bleed, it does nothing when you can't hit the boss 40% or 60% of the time. Or you spend too much time downed due to misttimed dodges, lag when you need to dodge, etc.
Likewise there are several monsters in dungeons that will turn on any player meleeing them very quickly and either 1 shot them (any hit will 1 shot the thief or close to it from these mobs) or drop a lot of ground target AE around the mob. This makes meleeing impossible. Don't forget, all mobs are not supposed to act the same. Some focus on ranged targets, some focus on whoever is meleeing them, some have random AEs with bugged animations, some have a continual PBAE..
And of course, lets not forget the power of low cost blast finisher spam from thief cluster bomb. Can lead to significant healing, chill, AE might, weakness duration, etc.
I could go on, about some bosses and mobs having so many HP that doing 13k or 15k or 10k doesnt make you kill it any much faster and imo doesnt outweigh the risk of being 1 shotted, but lets sum up my point:
All weapons are situational, and everything has its place. Straight out killing is not always better as straight out damage and relying on dodge is risky. Controlling a situation, staying out of death's reach while still doing damage is imo most advisable path. Thief SB is one way to continually do some damage and keep alive, kite mobs or transfer aggro. Regardless of 1 mob or 200. All dependant also on your team make-up in a dungeon or who is also around in open world combat. Just writing off a weapon or a skill as useless or not (as) effective without looking at the whole situation, combo fields, finishers, team make-up is relatively short sighted imo. This game isn't in general rock paper scissors/dps beats all/you always have class X with you (unless ofc you do).