So is there actually a reason to build Bloodthirster over Death's Dance on most champs?

So let's compare BT and DD real quick.

BT costs 200 more gold than DD. Depending on the situation this can be significant or not, but let's put that aside for now.

Both give 80 AD, so we can ignore their value as damage items. Let's look at them defensively.

BT gives 20% lifesteal, and can 'overheal' you to a flat shield value. 50 if you somehow get it at level 1, up to 350 at level 18. If you stay out of combat for 25 seconds this shield will start decaying, though realistically speaking it's safe to assume you'll have it up most times you go toe to toe with a champion.

DD gives 15% lifesteal, and cannot give you a shield. From a glance, this sounds weaker (5% less lifesteal, no 'overheal'), but let's take a closer look now.

• BT healing only works on auto attacks. DD overheal works on all physical damage (with 1/3rd effectiveness on AoE, but still better than the no-healing you'd get from BT). Sivir Q, Darius Passive, every single ability Riven has, you name it, if it's physical damage DD will heal you for it. While I admit this is champion-dependant, this essentially means physical ability-based champions will heal for far more hp with DD than with BT most of the time. Someone correct me if I'm wrong but I believe the reason Draven rushes DD instead of BT is that BT won't heal you for the Q bonus damage?

• DD's passive is an incredibly strong pseudo-tank ability, assuming you have the time to swing back. 30% of the damage you receive being delayed combined with DD's inherently extra-effective version of lifesteal has the potential to give you far more effective HP than an at-most-350 HP flat shield, because on top of getting healing from your abilities, the damage you're taking is delayed, which essentially means you have more time to get your attacks out to heal you in the first place.

That last bulletpoint applies even moreso if you aren't a super squishy champion (Example of non-squishies: Darius, Rhaast). This is because with a champion that will have a lot of HP and defensive stats, a 30% reduction on top of all your resistances and HP just means you're taking damage even more slowly. Assuming you have time to swing back, it's essentially an even stronger multiplier to your resistances, but since it applies after your resistances, at worst you're just taking the same amount of damage you would have taken anyway, but on a delay.

So in summary: If there was a champion that relied almost (if not) 100% on their auto attacks for damage, I could see them possibly building Bloodthirster over Death's Dance, but even then an argument can be made for effective HP with the % that gets delayed vs having a flat shield, so I don't see and reason the majority champions would take Bloodthirster over Death's Dance.