When used properly, they're game changers. Unless you're in a city, they'll almost never win a straight-up fight with an armored unit. That's not what they're there for. They're force multipliers that make things easier for your mechs and tanks.
They're the cheap escorts that make enemies wary of trying to get inside the minimums of your assault tanks.
They're the guys that make that stand of woods just enough of a pain in the ass to secure that your opponent decides not to bother with it for now, making it safe for your mechs to turn their backs on it.
They're the units you hold until last in your initiative order while you pointedly open the rulebook to the Anti-Mech Attacks section while the other guy is moving his stuff, psyching him out and putting him on the defensive even when he wins initiative.
They're the best IDF spotters in the game, period. An Archer pilot who doesn't budget for at least one platoon's worth of beer after every battle is an Archer pilot who doesn't really like kill markers.
Mechs may be the kings of most battlefields, but if your fight wanders into a city, they're nothing to the foot troops. Even kings must kneel when confronted with gods.
Easiest ambush ever. Remember that infantry are cheap, can be hidden in almost any kind of hexes, and Beagle Probes can't spot them. I'm not encouraging you to flood a mapsheet with guerrillas or some similar dick move, but nobody likes to be ambushed. All he needs to do is stumble over a couple such ambushes, and you can make him so paranoid that his advance will slow to a crawl.
Speaking of psyops...the perfect exemplification of frustration is a big energy/Gauss mech like a Marauder-3D or Devastator trying to advance in the face of indirect fire called down by a single dug-in foot platoon...who is in range. These mechs are used to being the boss monsters of the battlefield, they do NOT like being confronted by a tiny foe they can barely scratch.
And before the inevitable "but" comments surface, I know there are things that kill infantry dead in job lots. I know of these things, you know if these things, we all know of these things. If you incorporate infantry into your plans but don't have a plan to kill the infantry-killers, what happens in that game is 100% your fault.
Long story short, to paraphrase myself from a long time ago: If your infantry operations cannot be described as shenanigans, you're doing it wrong.