Constructive, got it. How about point by point?
"Honestly for the cost and effort to get this guy, not totally worth it in my eyes. "
--That's fair, but we're not here to argue opinions.
"It's Regen is obsolete because it runs away from damage."
--So the fact it actually has 150% health is worthless? C'mon man, you'd die for a Golem with that. More on how this isn't useless later.
"And if it does get caught out it dies in an instant. "
--Not true. While most of the time it will die rather quickly when completely surrounded (why the hell are you deploying it in the middle of their troops?), if you use it as a stop-gap between, say, your base and an enemy wave, or your bots and theirs, it only has to live through 3-5 bot attacks, which its regeneration helps with, along with the fact that, as far as I know, it prioritizes converting units which are an immediate threat. This creates a buffer zone, thus protecting it. This works because the current programming of the bots prioritizes attacking enemy units before buildings.
"It's shield is also pretty useless for the same reason. "
--On top of the fact that it can be used as a buffer as mentioned above, it can also be a very large and long-lasting decoy if needed. I have used the Warmonger several times to distract a fresh wave (usually just the small ones) until my bots get done on a rocket or whatever and can come deal with the wave (see above about priority of attacks). The shield gives you, what, 9 seconds? plus the aforementioned 150% health, combined with the above sited "buffer zone" of converted bots, all comes together to make a very nice road block, or perhaps just a much-needed speed bump.
"It only has a single attack against bots"
--I'm confused why you think the Valkyrie's attack makes it worth it but the Warmonger's doesn't. Or are you only taking into account the Valkyrie's EMP drop effect when considering their efficacy? (I would ask if you thought the Valkyrie was better because of its reanimate effect, but you NEVER mentioned it in your Tier system, which is mind boggling because it's current reanimate levels are pants-on-head retarded and should probably be nerfed) You might be reminded that, sometimes, this game, and others like it, are all about hyper-micro strategies. There have been times where 2 bots made the difference. And when everyone's bots are maxed, you need every edge you can get. Conversion and buffs are the only way to accomplish this currently, and the Warmonger has both. So lets talk about the buff now, since you don't need to be convinced of the Valkyrie's efficacy when it comes to conversion.
"and it's Rage only makes an impact on very long term maps where you can't dodge"
--Let's ignore this flippant dismissal and actually list all the benefits of the Rage ability:
-> In a world where everyone is lvl30, you need a way to make your bots artificially better, or their bots artificially worse. Rage does this by boosting your bots speed and damage (and maybe damage resistance? don't quote me on that)
-> As for "making your opponents bots worse", most people go with the EMP option, and rightly so. an 8+ second stun can make the difference in a very close fight. But what if your opponent is relying on that? What if they "dodge" your wave and EMP with a Golem or Valkyrie drop? They have one titan between their base and maybe 15-25 of your bots. So just Rage them out of it. They're going to have about 5-10 seconds before their next wave comes, and even just 5-8 bots can take down even a hacked base in that time.
->What if you're on a crunch? What if you KNOW you need to get your "dodge wave" back to the main fight before bad things happen, but you still want that sweet 3-star victory? How about rage them for speed and damage boosts so they take those rockets down faster and get back before the next spawn wave, or before your opponents bots have a chance to deliver those little blue pixels of live-giving joy? Sounds good to me, and I have actually done this several times. It works, trust me.
-> Finally, we come to the one point you DID manage to list, which is the grinder map. Sometimes, you're only getting one or two stars. But hey, 20 ladder points is better than a wasted attack, right? So you need to just keep pushing stubbornly until you finally tip the tides in your favor and can push back. Get your Rage on.
I hope this has been educational for you and anyone else reading. Don't make the mistake most new players make and think it's about "all damage all the time". Sometimes RTS and the like require finesse, sometimes they require a less-direct approach. It's not always about kicking in the front door with two slashers with their (albeit impressive) drop damage and line nukes.