Dray,
Others will have different opinions of course
I would say it can add an interesting dimension in many ways. You get experience for getting the stronghold, you sometimes get equipment, and it adds several 'strong hold' related quests. However, it doesn't impact the actual game very much, the story line contines as it was regardless...
On a related note as to the thief stronghold.
(slight spoiler?)
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I found that after getting my thief stronghold, that I was only able to do the first stronghold 'task'. I (erroneously?) then decided to head to the save Imoen, which takes one away for quite some time. When I returned to Athkatla, I had 'lost ' my stronghold. When I tried to buy it back, it automatically scripted back to 'You have lost your stronghold' no many how many times I tried to get it back. So, I found it to be disappointing, but others might have had more positive experiences?