As the DM I am playing multiple NPCs, monsters, describing areas, answering questions, etc. It's very rare that I can immerse myself in a character, and when I do, it's usually when I'm playing a single important and/or recurring NPC that is currently involved in a fairly lengthy RP session.
As a player I am only ever playing one PC, so immersing myself is really easy and is my ultimate goal and major reason for playing RPGs. This is probably one of the main reasons that I am so vehemently against social skills working on PCs. To inhabit my PC to the point where I am thinking and playing a character that is unlike me, struggling with hard decisions that the PC would struggle with, I need to know and understand the character very well.
As soon as some mechanic tells me that my character thinks or feels some way other than he would think or feel, I'm booted out of immersion and it is an incredibly hard and lengthy process for me to achieve that same level of understanding with this new thing in his character, and such immersion will likely never occur again, since by the time I can do it, more social mechanics will have altered him further starting the whole process over again.
Social skills vs. PCs destroys one of the main reasons I play roleplaying games.