You Need a Mac Pro Now
The biggest reason to buy the current Mac Pro is that you need a new Mac Pro to do your job and meet your needs.
Maybe you've been waiting for the last 2-3 years and your current computing power isn't enough. It could be that you were looking to buy a MacBook Pro, but you're not ready to deal with dongles and adapters that you normally need to connect to a desktop setup.
This is actually why Apple announced the current Mac Pro. Phil Schiller told Daring Fireball,
"In the interim, we know there are a number of customers who continue to buy our [current Mac Pros]. To be clear, our current Mac Pro has met the needs of some of our customers, and we know clearly not all of our customers. None of this is black and white, it’s a wide variety of customers. Some… it’s the kind of system they wanted; others, it was not.
In the meantime, we’re going to update the configs to make it faster and better for their dollar. This is not a new model, not a new design, we’re just going to update the configs."
For a number of users, this could be the right option. If you can't play the waiting game, this is a good reason to buy.