And if I get this straight, the Poison spell wouldn't benefit in any way being Heightened (apart from the fact that it could break through a Globe of Invulnerability)?
That's right. It also means that you're not trading DC for extra effects with the other metamagic feats, and that you can amp up the DC on a wand or scroll just by increasing the caster level - something you can't do with pretty much any other spell.