If I play the game with 4 or 6 of my own characters and 2 or no NPC's am I missiing out on the Baulders Gate experience?

Yes.

Well i spose you'd miss out on heaps of side and sub quests wouldn't ya

You will definately miss out on alot of the unique flavor and character of the game, but some people don't really care about the intraparty dialogue and personalities. The thing you will miss out on the most will be most of the NPC related quests and subquests and the subsequent spoils and XP's that go along with them.