Is a Mac Pro price or speed justified? That is only a question each of us can answer individually based upon any number of factors, being it finances and expandability and less of an issue of performance. Afterall, if you need the performance, you probably already know you need it and such questions don't come up when you need the power.
My iMac 2.8Ghz is more horse power than I need, I have 4GB of RAM in it, more than I really need, and I have a second display attached that is a 24" display, giving me dual displays. I have two external 750GB drives attached via Firewire. I can easily add more.
To me, the iMac is VERY expandable. However, if tomorrow I had the sudden urge to get a 3rd monitor, I can't hook it up. If tomorrow, I needed 8GB of RAM, I can't purchase it and install it and be off and running. If tomorrow, I want to add or replace my internal HD's, I can't do it. If tomorrow, I need a better graphics card, nothing I can really do. If I need SCSI support, not going to happen.
For many people, the iMac is expandable for their computing needs. However, there are exceptions that I noted above. I know that if any of the exceptions come up for me, chances are, I will be shopping for a new Mac anyhow, never-mind the fact that I have no choice but to do so.
I think if the question is, "Do I need a Mac Pro", chances are, you don't. The key word is "need". Now, when it comes down to "want", then if your pocketbook says it's okay, then more power to you, literally.