Hello fellow Chaos Tainted Daemon Worshippers,
I stumbled onto this thread today and thought I would offer the OP my take on the do's and do not's of the lesser daemons and who they are good against. So to begin:
Bloodletters: These Khorne daemons are gnarley in close combat. As you would think with Khorne, they excel at taking down Space marines and other SMEQ (Space Marine Equivalent) units. With WS 5 and S 5 on the charge they can easily take down a tactical squad in one or two rounds of combat, especially with AP 3 hellblades. Bring these guys if you think you will be up against Space marines or other infantry that has a 4+ or 3+ armor save. Also, would recommend taking the Bloodreaper character with 1 lesser reward. This is because you will be able to challenge out and possibly kill their power claw/fist wielding character before they can attack. And with Devastating strike you have the possibility of instant death. That'll make even a Space Marine character in terminator armor think twice about challenging.
Daemonettes: These Slaanesh lesser daemons are once again good in close combat and are also very mobile. With the ability to run an extra d3'' they can get places on the table much faster than other god's minions. Also, like Bloodletters, they will be attacking with a high weapon skill, but will be striking at init. 5. Therefore, if your opponent has a heavy, slow, terminator force then daemonettes are the way to go. You will outmaneuver them and will also have a very high chance of rending his expensive troops to death with 9pt models. Also, equipping them with a standard will allow the daemonettes to survive a round of combat. With the WS reduction to your opponent, sometimes reducing them by 3 WS, they will often be hitting you back on 5s which will greatly increase the survivability in combat of an otherwise squishy unit at only T 3.
Plaguebearers: Being a Nurgle player, I really love these guys. With the only major drawback being that they can't run due to slow and purposeful, they are much "stickier" than the other lesser daemons since they are rockin' out at T 4. I typically take two blobs of Plaguebearers (15) with a herald of Nurgle with the Greater Locus of Fecundity (Feel no Pain). This will give the Plaguebearers essentially a 5++ rerollable unless they get hit with S8 or higher. With this kind of setup you are looking at potentially a very durable unit that is great at holding objectives in the back which your Khorne and Slaanesh daemons charge forward. Furthermore, these guys are your big, cheap anti-armor. With the Touch of Rust ability on their weapons (auto glance on 6s), on the charge these guys can potentially even take out a land raider, decked out dreadnought, or any other armor that is giving your army problems. So plan on taking a few squads of these guys if you think you'll be up against a heavy armor list. A great all around unit that I would highly recommend taking.
Pink Horrors: Although I'm not a big Tzeentch fan, these little gremlins can still pack a punch via their psychic powers. A great unit to take if you intend to be doing a lot of casting in the Psychic Phase, since their flickering fire ability gets even stronger the more warp charges you pass the test with. With potentially 4D6 shots, just a squad of 10 pink horrors can potentially put out in one Psychic Phase 24 S5 shots, with the potential to inflict additional wounds if the enemy unit fails their Toughness check. Like Plaguebearers, these guys are meant to stay in the back and hold objectives. Also, don't feel like you can't move them around to grab objectives in Maelstrom games since all of their shooting is done in the Psychic Phase. This means that they can move and then run in the Shooting phase. So, if you feel that you will need some ranged support for your front lines in your list then Pink Horrors are a solid take. Just be sure to protect them, since they can't take a charge very well.
I hope this answers your questions about which lesser daemons to bring in which situations and a brief overview of how they perform certain roles in your list. If you would like me to go over the greater daemons/elites/fast attack choices of the various Gods let me know.