An interesting thing is combi-flamers, as mentioned by theharleqwin, can be used as an assault deterrent. Charging a bunch of
WG terminators who will hit you d3 times per combiflamer before counter attacking with claws, power/frost weaps, fists, whatever they have its going to be a nice melee weapon.
WG terminators make the best "tactical" terminators
IMO due to the customization that's available, plus good wargear for good points- combiweapons and the terminator only
HW's, and nothing worse than a powerweapon for base equipment. Combiflamers can also be great against dark/normal eldar. Lightly armored armies that can be costly(thus small) and require synergy do not like whole units being removed very quickly and easily by combiflamers.
Combiplasma's are very very nice to have in term squads. Being able to put effective (armor denying) fire on heavy infantry is only achievable for terminators via combiplas (for the price of that 4 shot rending autocannon, i get 5 combiplasmas that shoot either 1 or 2 shots each.) Unloading 2+ combiplasmas into a key hard unit can really give you the edge over it. For example, a hive tyrant- big, scary, nasty, tough, is a sucker
MC since he lacks and invul save. If you catch him out of cover with 4 combiplasmas, he's toast. Daemon princes and Greater daemons, Enemy terminators, plague marines- tough hard to shift units can be cut through quickly by a plasmagun in the right place, and terminators are tough enough to get there.
combimeltas are less required than in 5th, but I see their main use as transport openers and dread insurance. A unit in a transport- you need to assault the transport, weather the units fire, hopefully reach them next turn after they move away and you cop overwatch fire and then you get to grips with them- assuming you blew the transport in the first case! A combimelta will open that can to let you attack the delicious juicy contents. Against walkers, such as dreads, a combimelta will help avoid melee against the dread. Essentially- dread swings first. Hits on 4+, wounds on 2+. I hope you got a storm shield, since 5++ on a 40p model ain't

. You then swing back, hit on a 4+, glance on a 4+, and hopefully blow it with your fist... its not as bad as 5th, but a pair of combimeltas is a cheap way of avoiding a potentially costly melee. I mean, you'll more than likely win (with enough fists etc), but your going to lose expensive models. 10p can potentially avoid this.
The bottom line is combiweapons are very situational. If you find bolters aren't biting into enemy heavy units enough, combiplasmas can help. If you find your small elite force being swarmed by mess of light infantry, or are facing the light but deadly D/Eldar, then combiflamers can be a boon (especially vs orks with that overwatch fire). If you find people are still stuck in 5th ed mentality of "mech mech mech mech!" or want cheap dread insurance, combimeltas are for you.
HTH