I'd have to agree with the logic that if you are asking on here, you problably don't need it. But ask yourself, what would you use it for? If you do most of your home browsing on your MBP, you probably don't need it. Do you play many game apps on your iPhone? Do you do any e-reading? If so, then you may want to consider one.
For me personally, when I am just browsing, I am on my MBP. I usually only "play" on my phone when I'm out and about, which is only for short periods of time. For the first two years of its existence I truly had no need or use for an iPad. When a basement flood destroyed most of my books and almost got my photos, I decided to go digital with both and mine serves mainly as my books and photo albums. And it has also become my planner/organizer.
Could I live without it? Absolutely, but since I got it, it's been awesome, especially on a couple of trips I've taken. More than likely, if you did get one, you would just spend less time on your laptop and phone.