I haven't really been thinking about what a balance tool the blood points represent. Or I should say, I was under valuing them.
I think it's fascinating that in a shooting focused edition, there is now a mechanic to add additional units to the battle. Also interesting is that there can be diminishing returns, as more things die there are fewer potential blood points to earn. But if you summon units they can also earn more blood points by dying or killing.
It really is an interesting dynamic: you're actively throwing scraps of squads into combat in an effort to kill them off. Oh, just the biker champion left in that squad? Well, he can soak up Overwatch from that big enemy blob so that your Flesh Hounds get in untouched, and if he survives, he can issue a challenge for the shot at yet another Blood Tithe point--two if he gets killed in that challenge!
It's really a different dynamic, where rather than attempting to tarpit with scraps and the like, or running and hiding on an objective, you are actively dashing your units on enemy rocks that can crush them... while your new summoned unit goes and gets that Maelstrom objective card you needed instead. And your Slaughter Cult gets free FNP after you summon that squad, to boot.
The ability to redeploy your army at a whim, bringing in fresh units to threaten backfield objectives or contest/claim important objectives, and getting these semi-regular summonings off, it A+. It's a really clean execution of a very Khornate mechanic that gets players thinking in a very, very bloodthirsty way: it actively affirms the truth that Khorne cares not whence the blood flows, only that it does. You don't replace all of your losses, but you do have a constant influx of fresh, cheap units, and all of your units have great charge bonuses. If you have a very MSU list, then enemies often have assaulted one of your small units (perhaps wiping it out), then you pile in with three-odd small units whose assault bonuses really stack up: say, a squad of 3 bikers with Hammer of Wrath with Shred then also Rage, and a squad of 5 Flesh Hounds with Hammer of Wrath and Furious Charge, and a squad of Possessed with Furious Charge and Rage. That is a serious amount of pain just from raw additive stacking of what boils down to the occasional +1A and +1S here and there. It makes me appreciate the value, too, of the 2 Blood Tithe reward bonus: +2A at S5 per model assaulting, no matter if Daemon/marked? That will chop right through anybody you need it to!
Speaking of assaulting with lots of mobile small units: I am a firm proponent of Daemonkin as a highly mobile MSUspam army build--always at least one Gorepack, basically--which means I only want to play it on Maelstrom missions, since 1/3 of the time, I would lose Eternal War missions in the "roll to select mission" step: when The Scouring or Purge the Alien mission is rolled, given how many Fast Attack choices and kill points I'm bringing. Fortunately, having a deck of Daemonkin datacards means that I can hand off my old standard-rulebook set of datacards to anyone who lacks such a set, which is usually the stumbling block that sets me playing Eternal War in my games.
Fearless Daemons not having to worry about Instability and making even MSU squad sizes pretty hard to shift, locking up enemy units, is also stellar.
I'm actually impressed as to how well it all works together, from my limited playtesting and observations thus far. It's pretty gratifying, honestly: I haven't had a positive opinion about Khorne, apart from Kharn, for going on two editions since Berzerkers lost their bonus attack in favor of Rage. But now we can Blood Tithe them that +1A, along with free Furious Charge on top of their Rage since they're part of the Slaughter Cult, so each Berzerker gets 5 S5 WS5 attacks. Not bad, not at all bad for a Slaughter Cult tax core unit.
Disposability, interchangeable solid melee threats. Meltas wherever able to take out tanks. High mobility, reinforcements every turn or two wherever they're most needed. The Blood Cult (and its nested Slaughter Cult, and its optional formations) are great fun. Free Bloodreapers on your summoned Bloodletters means more Blood Tithe points from challenges, or enemy characters sitting out kicking dirt.
My real concern is that the mechanic of Blood Tithe points is scoped pretty solidly for a certain size of game that won't scale above 2000 points or below 1000 too well. More playtesting to do!