I personally don't like Lesser Daemons...
Cough, Plaguemarines, Cough
Anyway, Lesser Daemons are primarily:
-Cheap, spammable scoring units
-CC support for other marines in combat (same stats as a assault marine in combat, except for armor save)
-Fearless, so are great tarpit unit, esp. weapons skill and toughness/str of marine.
-Fearless, so are great at taking fire off the marines if the opponent shoots at them.
-Are tarpit/cheap, useful to cause target priority checks.
SOOO, a unit of 20 would be godly good at taking on swarms of GEQ in combat and being the worst possible tarpit for anyone...
1. you must deepstrike them from a Icon... 6" away, so arranging the 20 daemons might be really annoying, also, you might just end up killing your own daemons when there isn't enough space to 'complete the ring'.
2. asking for odinance/rapid fire...
3. most likely out of assault range
4. in difficult terrain.
5. Is a huge investment on just one unit... 260 points, it might not seem like alot, but this new codex is about squads of 10 and other units are around the 260-290 points range, IMO.
2 squads of 10 would be good for CC support
3 Squads of 7 (+1 more daemon) for a mix of scoring units and CC support.
4 squads of 5 would be good for scoring unit spam.
My 2 cents. Good Luck with them.