Not for development they shouldn't.
For testing sure, but are you really going to justify that the development process should take at least twice as long because of poor QA?
I really question whether a fancy machine helps with productivity. Some things are obvious, like plenty of screen real estate so you can switch contexts just by flicking your eyes. But most of the tools are just gratuitous power hogs. Rational Rose, Eclipse, etc. Let's be brutally honest, modern IDEs don't do much that Energize wasn't doing perfectly well well over a decade ago, or that Emacs was doing a decade before that. I could do my job on an old SparcStation 5, and I bet everyone else could too.
More specifically I can't without taking a huge hit in productivity (and there are somethings I wouldn't be able to do at all on a Sparc station).
In order to do my job, here's a short list of what I run on my MBP:
Eclipse (explaining why Eclipse is better for my needs than emacs (which I used to use) would take a while, but lets assume I'm actually more productive with Eclipse)
Parallels Linux VM running Oracle 10 XE
Parallels Windows XP VM for testing with IE and IE 7, and running a couple of windows only tools I need on occasion
Mail.app
Adium
Safari
Firefox
iCal
iTunes
TextMate
OmniFocus
VPN Client
SQLDeveloper
JBoss + ATG Commerce JVM (768 MB of RAM)
JBoss + ATG CSC JVM (768 MB of RAM)
Ant builds triggered from Eclipse that compile hundreds of Java files and deploy 5k+ files to the various JBoss servers
The JVMs have to be restarted frequently to pickup code and config changes
While I'm sure there are lots of people who could do their jobs just fine on a SparcStation 5, or any old computer, there are some people who need large amounts of RAM and very fast CPUs in order to not spend their day waiting for builds, JVM restarts, and VMs to come out of swap, etc...