Munchlax isn't the worst in-game partner by a long shot (Cheryl's Chansey anyone? Buck's Claydol and Dawn's Clefairy are not exactly helpful either). Leer spam helps you get past the Bronzors, which, depending on your team, you could be having a lot of trouble with. Besides, you can always kill it if you want a new partner.
It just needs Follow Me. As does every in-game partner really.
Honestly, I'd really say it is - at first glance, compared to the ones you've listed, it might not be. But it also has to be seen in context. Cheryl's Chansey and Buck's Claydol don't matter since you're literally just curbstomping everything you come across with ease; there's no major, tough battles and you don't care about the Pokémon you're partnered with because what really matters to you is the free Pokémon Center after every battle.
Tagging with Barry at Spear Pillar is a completely different situation. It's a boss battle, and admittedly a rather tough one at that. You're up against two admins with Pokémon that are either very strong or very annoying. There's no grunts or wild Pokémon to go through and there's no forced battles right after - if you're playing Platinum, which you should - so you don't really need a heal. So aside from tagging with Barry again at the Battle Zone, this is the only tag battle in the game where it actually matters what Pokémon your partner chooses. And they choose, for whatever reason, an annoying little bastard that does nothing but sit there and stockpile/swallow the entire fight, and MAAAAAAYBE it might OCCASIONALLY use Screech or Body Slam. Oh yeah, it's one attacking move is resisted by the first Pokémon sent out by the admins, great. And it doesn't even go down quickly because of it's defences and the Bronzor's dire attacks, gods. You need to specifically target it for it to go down at all until Barry finally sends out SOMETHING that can do some damage.
And hell, look at it atmosphere-wise. The world's about to collapse around you. You're up against two admins which have caused you nothing but grief the entire game and it's time to take them down as the last line of defence before facing the psychopathic Team Galactic Boss and the Legendary Pokémon he's summoning. If there was any time you needed help, it's now - and right out of nowhere, your rival and best friend shows up to help you! This is great! Let's kick some ass and save the world! ... then he sends out this little bugger that sits there the entire fight and does just about nothing to help you. Thanks, Barry. Oh, fight's over, you're going home now? Sure, that's... that's cool. Just leave us to fight the boss. Your contribution will be remembered. Couldn't have done it without you.