Plasma Pistols are a purely fun upgrade - they are way too expensive to be competitive but you are playing Dark Angels so the more Plasma the better, right?
Melta Bombs are definitely a nice grab in case you ever need to charge a Dreadnought or
MC - and a lot more efficient than the Eviscerator, which is a very expensive upgrade. But generally people will take two Flamers and either a Combi-Flamer or Power Sword and use them to clear infantry - it's fun to play and very points efficient.
Generally they also stay at minimum size to get the most efficiency out of their special weapons, but taking ten men is always fun (though again, not competitive).