Adding a Forum to my site: build one from scratch or use existing open-source platform?

You seem to have a good understanding of why you should and shouldn't write your own forum software so I'm going to focus on my opinion instead of hashing out facts you already know.

Forums are favorite places for spammers to spam. They're right up there with blogs. Even existing forum software have a hard time keeping up with them. If you roll your own forum software you may find keeping up with them a full time job and not worth the time and effort.

Existing forums software offer a familiarity for users. There's really just a handful of popular forum software out there and most people who are active in communities have seen most of them. If you use one of them in your site your users with be comfortable using them right away.

Existing forum software offers you a chance to get your community up and running quickly. They also offer you a chance to add a lot of features quickly.

The cons would be a lack of "perfect" integration with the rest of your site. But the question is, "How much is that offset by the positives listed above"?

I'm in the habit of reinventing the wheel sometimes, 'cause u know what you are doing, It will be light weight, definitely lighter than any forum because they a far too much features that nobody use.

It's worth to spend sometime and design your own. . . .

General forums have bugs which is known and someone can attack u, but if u have your own forum then if somebody needs to attack, then he need to do a lot to know your forum's flaw. Fully compatible with your site.

and at last periodically add some feature that your users need.

I would run away from rolling your own forums. It will just be a headache in the long run. Please take a serious look at vBulletin for powerful forum solution, or vanilla forums for lighter weight package. Both have a robust plugin API which will allow you to integrate with your current site. Both have a very nice theme system, which can make the forums look a integral part of your site. Don't be afraid of all the features of vBulletin, eventually you may want to start using them.

For an example of vBulletin integrating with a website, checkout Bio Ware's new Star Wars forums. (Running vBulletin)