Multiple Instances Of Oracle Or One Instance With Multiple Schemas
Even if you don't have name conflicts and could easily combine all Dev environments into a single instance, I still prefer multiple instances for my Dev environments due to the following:

You really should have your Dev environments closely mimic your Live environments in terms of hardware, resources, and volume of data as much as possible.  You will be in a much better position to:

1)  Stress test new code before large (or even small) projects go live.

2)  Your Dev teams are far more likely to catch performance issues before they go live (even if you don't stress test).

Performance issues should be caught long before you go live.  The best way to do that is to test in an environment that is as close to your live environment as you can possibly make it (even before that code is uploaded to the QA environment, if you have one).