There are a few reasons I would use enforcers:
* They are great damage soaks - being able to repair 36 hull/second for 20 seconds with a built-in ability (via Reintegration) is nothing to scoff at, and can keep your fleet alive for much, much longer.
* They are usually the best unit to counter enemy light armor targets if you can build them, in the mid-game. You say "HC's are supposed to counter LRF, they don't" - I think they do, in equal supply amounts, especially with Reintegration, and especially if fleets aren't that large yet.
It sounds like you would prefer to use Assailants with their phase missiles. While phase missiles are quite nice and are the Vasari's strongest weapon, especially against enemy capital ships and starbases, building proper countering units to enemy fleets is somewhat more effective.
If you have the capability to research and build Enforcers but rely only on Assailants (and maybe Sentinels), you'll be very weak to masses of enemy flak - you'll probably have a somewhat hard time making a serious dent in their flak. Enforcers, on the other hand, will be virtually impervious to enemy flak, do significantly more damage against enemy LRF than any of your other options, and also are pretty difficult to destroy due to Reintegration.
Enforcers (combined with a few Subverters) are also probably the most efficient unit to counter the enemy if the enemy builds a lot of antistructure or siege ships. They're definitely more effective than Assailants or even flak in that regard.
For the other races, this seems not terrible, but for the Vasari with their phase missiles on their LRF, what's the point of ever researching enforcers in the first place? Wave cannons are one of those upgrades that you never do (it's usually one of the last upgrades I do in SP). Do they have a real function or what?
Phase missile upgrades are great because they improve quite a few Vasari ships and make them much more effective at taking out large enemy targets. You're right that wave cannon upgrades aren't really worth it - just like many weapon upgrades of the other races, the cost is rarely justified by the improvement.
But the heavy cruiser's weapon still remains an anti-light and a decent anti-heavy weapon. Phase missile upgrades might increase the effective damage of Assailants, flak, and bombers by 30%. But just using the default Enforcer gives you a much larger damage bonus against enemy light targets, along with significantly increased durability (for a while, at least, before fights turn into carrier bomber spam).
Enforcers are also a bit faster than Assailants. And a single Enforcer guarding neutral extractors can fight off a good number of enemies, much more effectively than any other ship I think, unless your opponent sends a large fleet its way.
