You don't NEED strut bars. But, they will help stiffen the chassis. Think about it, your car looks like (well where the shocks are) an open box like this

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That is not structurally sound. The bar links the top like this

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This is stronger and better for handling, since with an independent suspension car, the suspension loads go through the chassis of the car, so a stiffer chassis means your suspension mounting points aren't flexing and moving around, meaning your suspension can better do the job it was designed for.

Now, I would only get a strut tower bar that you CAN NOT BEND BY HAND by pushing down on it in the center! I've looked at most strut tower braces, and you can push on them and bend them with your hand when installed. In my opinon these are worthless junk, just touch up pieces for the engine bay. Get a REALLY stiff one you can't bend AT ALL. They are out there.

Yes, I know SOMEONE will argue the suspension load goes through the length of the bar and pushing on the center and having it flex there doesn't mean it won't work. Well, let's be realistic. How strong (even if the force goes through the length of the bar) can it REALLY be if someone can push on it and bend it? Do you REALLY think it will actually do anything significant? Maybe, but I doubt it! If you're going to put a bar in there do it right the first time and get one you can't bend. It should be a nice solid tube with a strong attachment to the chassis.

And, the bars are worth it, the proof is they are showing up in new cars now. The new 350Z has them front AND rear (even though it interferes with the rear corgo area, Nissan still put it in the car, made it nice looking too) and they are in the new V6 3G eclipses. Mitsu is cheap, they wouldn't put it in if it didn't do something :)