For me it's all about sheer power and flexibility. For one, eliminating and freeing up some precious desk space with four hard drives inside my computer is nice.
Unless you have the cash burning a hole in your pocket for a Mac Pro, it is the money that matters first and the other stuff is secondary. You have a debt load and no solid job right now. That doesn't sound like you have $3,000 in purely discretionary funds.
Second, from the initial description it sounds like this is
not your primary work computer. If the intention is to get a job "in-house" somewhere the computer located there would be your primary work computer. This would be a computer where you might be doing secondary work on. I wouldn't create a plan where your new employer leaves you with copious spare time to do something other than what they pay you to do.
If striking out as an independent contractor where at least the majority of your work is done on the new Mac Pro then that would be a different story. Again though your other costs ( bills , necessitiies ) versus what your cash flow would be primary factor.
Third, long rendering time aren't necessarily a problem if have something else to do. A 4-6 hour render will be done if start before going to work. If you had to do multiple things at the same time on one box then the Mac Pro has more leverage ( render and music and ....). However, if can leverage multiple boxes for multiple workloads then have a reduced need for one more expensive box.
Back when I used PC's I would just upgrade a component when I needed to, instead of buying a new computer and monitor ever two to three years and it seems the Pro is my only option for that route.
Not the only option. You can decouple the Monitor with a Mini also. Nor it is absolutely necessary to upgrade every 2-3 years since the machines you have now are over that time range. Nor have you factored in offsetting trade-ins savings/costs.
Do you guys think the MP offers more longevity in your investment?
This is where some folks get into trouble. It is not a linear increase. A Mac Pro that costs twice as much will not get twice as much support/longevity. Ask G5 users if they got a "get out of jail free card". Apple gives around 7 years before the machine is likely to drop on to the "legacy" list. You may squeak out a few more years with a Mac Pro but that isn't a good foundation for a debt repayment plan.
I know you spend more upfront but do you get more out of it in speed, efficiency, total time of ownership, and upgrades?
Upgrades are an additional cost. So if the machine "pay off" period is 5-6 years it is 5-6 years till can buy upgrades without backsliding on associated debt.
but to generate revenue it makes me nervous to use an Imac. Because if any little part goes bad I could be out of work for days or weeks even.
As others have pointed out that is a bit long, unless you don't have a good Apple repair shop nearby.
The original post mentions a MBP that is coming due for an upgrade also. If you sink all of your Mac budget into a Mac Pro you will be creating the single point of failure. With two Macs, you have a fall back machine to go to.
If shifting all the mobile workload to some other device (iPad , iphone) then perhaps.
