Yes, you most definitely will be missing out on many quests that are only generated by the game generated NPCs. In my own single-multi-player games I usually will have 2 or 3 characters I generated, then add and remove game NPCs as their quests finish.
Everyone plays in their own style tho! If you want a quick run-through of the game just to see how it works, then using all your own characters would be a good way to do that. BG2 is one of the most re-playable games ever created! Different quests per class, and NPC.
Have fun!
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Do not meddle in the affairs of dragons,
for you are crunchy and good with ketchup.
(Most Happy Receiver of Zeeke75)