Short answer: Yes, you do not need to purchase a JBoss Developer Studio Portfolio Edition to use for Development.
Long answer:
JBoss Developer Studio (Free) and JBoss Developer Studio Portfolio Edition (99$) are two different things.
Meaning the product that costs 99$ and is named "JBoss Developer Studio Portfolio Edition" is for getting access to the productized runtimes (like SOA-P, EAP, RHEL, etc.) to test and doing development.
JBoss Developer Studio is a set of plugins and an installer supported by Red Hat if you have a subscription otherwise with Red Hat.
The output and usage of JBoss Developer Studio is not limited to be used togheter with a subscription - the artifacts you create with Developer Studio
is completely up to you and owned by you.