One of my group was commenting on how he thought it was no big deal to drop your essence down the way it was in cyberpunk. Are there any mechanical reasons to not lower your essence (through mods or whatever) as a mundane? I'd always thought it was kind of implied as an rp thing, but for some people have to be told to do it by the rules. Any thoughts/info?

In third edition you would be harder to heal, but that may not be the case in 4th. I also seem to recall some social mods but that may only have been for obvious ware.

For healing there are mods for the condition "Patient has implants" which amount to -1 DP per 2 points of essence lost.
There are no social mods for implants per se but the GM coud always pull dice from the pool of a chromed up sammie who's talking to a naturalist hippie of some sort. And then there's flaws like Cyberpsychosis giving -2 DP on all social tests.

VVVV Of course, I totally forgot that.

IIRC, there are also penalties to magical healing for lower Essence characters.

All I can recall in SR4 are the Magical and Mundane healing modifiers. In SR3 it was more built into the system that lower essence made you less human and that people could feel this from you. It gave you +TN in social situations. As far as I know, there are no -DP relative to essence in SR4.