Reasons:
- Given that work areas are so open, it is really obnoxious for a few people to gather around one person's monitors for anything. It is distracting to everybody else around them.
- Code reviews are much easier with a projector and a white-board. Switching between code modules while doing chalk-talk is inefficient using paper. Code review notes belong in the code, not on a piece of paper that will get lost.
- Your good developers will spend time outside of work doing "geek" stuff. You encourage that to be work related if you provide the environment.
- At some point, almost every developer will find that they are traveling, be it for a conference, an emergency customer issue, or to train a development group in Indiana. You've made them 100% inefficient by no providing a laptop.
- Needing to be 100% at the office to "work" is anachronistic, especially for development. Providing a mobile development environment means a developer can work productively for 3 hours at home before his dentist appointment instead of taking 1/2 day off of work. That means better productivity and happier employees, since they don't have to waste leave for life-maintenance.