Some would argue that they chew up too much supply, cost too much and the down-time is just too high. This is not the case especially in ZvZ, which is a very trade-heavy, push-and-pull matchup: you are never maxed in ZvZ. If you are, you aren't attacking enough. Also, ZvZ is very anti-deathball once lurkers are out. In a deathball meta, these kinds of units would be terrible, since it subtracts from your own deathball and the utility is just not worth it. But in a game where battles happen all across the map, and it is constant unit-trading, it is possible to slowly add on swarmhosts. You will be a bit behind at first, but they will quickly be worth it after 2-3 locust volleys if used right. The damage and damage buffer that the locust provide against lurker lines is invaluable: the enemy is forced to take a very unfavorable fight, or abandon the position (very important in the new positionally-based metagame). The swarm-host acts as an earlier, mineral based siege-breaker as opposed to a later, gas intensive viper or a less reliable ravager. Proper swarm-host usage is also a skill players will learn: in order to make the swarm-host useful, you MUST use those locust every 47~ seconds or else its a wasted investment. And making too many swarm-hosts (probably) makes you have too weak of a mobile ground army, and the enemy would probably just be able to steamroll you. And too many swarm-hosts lategame seems too clunky to be used efficiently. Do you see the cool, unique and niche unit that is the LoTV swarmhost? What other units are as unique as it? The metagame will dictate it's exact uses, but so far it seems to be an incredibly designed utility unit. ZvZ is shaping up to be a very challenging, deep and fun matchup to play.
EDIT: I haven't tried it out against protoss ground or mech yet, but I can't imagine it would be as good. Also, I think colossi will get a resurgence in the metagame against bio and maybe Z. I theorize that protoss players will eventually split up their armies more to deal with the multi-pronged attacks of bio and zerg. Because disruptors are much weaker against lower unit counts (big hits much less guaranteed), colossi will provide an important, reliable source of AOE damage. Probably 1-2 per army, since you still want some disruptors.