Well, you are right on the first part. Most headphones will sound softer. You will be able to hear it, but it's soft.

That also basically answers your second question. But also, some people like to use headphones besides the extra audio out in live situations, so you can pre-listen and sync up.

So I'm about to mod my first DMG (waiting on the parts to get in the mail) with a backlight.  I also wanted to add internal prosound that comes out through the built in headphone jack so that I don't have to drill any holes on the case.  I know how to wire it, but I have a couple of questions regarding it before I do it though:

-After doing this, will the jack still be able to provide enough power and volume to drive a pair of headphones like before?  To my understanding, the mod will actually increase the volume, but I need to still be able to plug in headphones and I'm worried that the increase in volume will decrease the power necessary to drive the headphones.

-If you CAN still use regular (non-amplified) headphones on it, why do people, more often that not, add a second internal jack?  What is the benefit (other than a higher-quality jack) over simply replacing the included one?