Good question -- although you've already guessed what the answer is likely to be.
Not much of a point to running hdparm because Linode is (currently) a UML environment -- which makes device access virtual.
The "disk device" is, in reality, just a file on the host system, rather than an actual physical device with knobs you can tweak (DMA and stuff).
So... no, no benefit to hdparm on a Linode. I also run Gentoo, and it's not a problem to remove it, even if it screams bloody murder about it.
It's there because the systemwide profile mandates its presence. (Gentoo normally runs on non-UML hosts, so this is a reasonable choice. Just doesn't make sense with Linodes.)