I stood before the same decision few months ago and I decided for a tri specific bike. My decision was based on few things:

1. I knew I'll be doing mostly triathlon and not much road racing
2. I decided to do half ironman this year and I did not want to kill my legs on the bike
3. The deal I got on the bike was very good

Generally I've seen recommendations to buy road bike if you do many group rides and road racing. But my view is - if you plan to do triathlons a lot you will benefit most from the tri-specific bike. Especially during the run you will be glad you did the ride on tri-specific bike.

Maybe. Read this thread, which discusses the study that was done on that question:

http://www.trifuel.com/forums/showthread.php?t=5768&highlight=geometry+study

I bought a road bike about a year and a half ago just to get one and start working out on. At that time, I wasn't sure what I was going to use it for but thought a triathlon or two would enter the picture. A year and a half later, I'm glad I made that decision.

First, generally road bikes can be found at cheaper costs than entry level tri bikes. Second, it took me a while to adapt to road riding positions (hoods, drops) and after adpating to the aero position this summer, I don't think there is any way that I could have just started in that position.
Third, I have converted my road bike to a tri bike, essentially, by simply moving the seat position as far forward as it will go, purchasing clip-on aerobars, and having myself fitted at a LBS. Now, this is obviously not the most advanced aero position, and my bike only cost about $1000 so the components and materials aren't top of the line but it has definitely been getting the job done.

Lastly, it's nice to be able to use the road bike in group rides and even bike races...TT/Tri specific bikes don't allow for that option. Group rides are awesome because you can really cover some miles at faster speeds than you otherwise would solo and bike races are just exhilarating and tough!

I think you're likely to get a lot of varying opinions on this one, and I think that points to the fact that this is one of those questions that can only be answered by how you feel.

The points you bring up against the tri bike are good, and honestly my tri bike doesn't see as much time as my road bike for some of those reasons.

Personally, since you already have what I'm guessing is a good road bike, I'd go for the tri bike. It definitely can make a difference in races, and it's nice to have the option in training.

Just make sure you get one that fits well. A poor fit on a tri bike results in discomfort, just like a road bike, but it also results in bad position which is bad for aerodynamics, which defeats the whole purpose.