This is not totally correct. SPL is the secondary program loader aka bootloader, like bios on a PC. It is what bootstraps android. Without it Android won't boot. But, ASFAIK, on the MT3G there is no need to flash a new SPL, so you don't have to worry about it.
People who flash new SPLs tend to want an engineering SPL so they can use fastboot commands. Or if they have a perfect SPL, the previous rooting methods for 1.6 and higher required flashing a new SPL. But this is no longer the case with universal androot.
What you are thinking of regarding the G1 is the "Haykuro" or "Danger" or "Death" SPL. There is nothing at all dangerous or deathly about this SPL in the context of the Magic/Sapphire/MT3G.
It is simply an HTC magic Engineering SPL (1.33.2005) that happens to work on the G1, rewriting the NAND partitioning so that larger ROMs can fit on the G1 by increasing space to /system at the expense of /cache (IIRC). Any Hero-based 1.5 ROM requires it, as do the later versions of CyanogenMod. CM4 did not require it for the G1.
It becomes dangerous only when in the state of being flashed, or being the subject of consideration with regard to flashing, on a G1. It is incompatible with certain radio versions on the G1 and flashing an incompatible radio with 1.33.2005 on a G1, and only on a G1, WILL brick said G1.
sources for further reading:
xda-developers - View Single Post - Magic SPL HBOOT collection
xda-developers - View Single Post - Booting Recovery Image Freeze
Magic SPL HBOOT collection - xda-developers
HTC Sapphire Hacking 2 Continue to HTC Sapphire Hacking - Part 2 - XDA-Developers