One of the mistakes I think I've been making with list building lately has been making the assertion that infantry are always in cover (which by itself is a fair statement if you play against competent players) and thus deciding that any high ap torrent weapons don't have a place in my lists. I think many people make a similar assertion. Especially when reading online, you often see people playing the cover card everywhere.
Things like plasmaguns (maybe even, dare I say, hot-shot lasguns) have taken a nose dive in popularity, but still have a place in armylists. Using a plasmagun to downgrade a 3+ to a 4+ is not very worthwile, and paying more points on your warrior acolytes for weapons that actually are just as effective against marines in cover as boltguns may seem just stupid. During gameplay you might not realise how useful they are. You're forcing your opponent to use cover. When playing against a list that doesn't really have a lot of high ap shooting (maybe just a couple of missile launchers) I'm not gonna use cover, I'm gonna maximize my range and rather move the sure 6" forward. The best spot for my unit might not be next to a crater. Situational, but just a couple of plasmacannons or plasmaguns might make movement more difficult for your opponent. He might pick cover and your plasma weapons won't do much damage, but you're already winning.
Some armies do this more naturally then others. lasplas razorspam already has that inbuilt plasma. Although I would rather take the plasmarifles on my broadside team leaders then crisis suits, a lot of tau armies have fireknife suits. On the other hand, a grey hunter spam SW list might not have that same kind of high ap torrent (but rather missile and melta which would be better off pointing in some other direction). Although grey knights don't really need that much of a buff, a couple of triple plasma warrior acolyte groups might be good just as a threat to your opponentm keeping their movement in check.