RPGMemes

Keep in mind that that "half must succeed" rule means the rogue is very slightly more likely to succeed with a noisy partner than alone...

That would depend on the DC. The rogue very well could have a 100% chance to succeed alone.

So what, exactly, is the justification for how a rogue "covers for" a plate wearing paladin with no dex bonus?

The rogue would be guiding them, keeping watch for enemies, watching out for hazards, etc. Maybe they'd help the Paladin pad their armor to make it more quite, or give them something to cover it up with so it isn't shiny. Sneaking encompasses the whole range of stealthy actions, including preparation and movement, most of which can be assisted.