These are great to do, for many reasons.
- Getting used to debating your research (useful prep. for the viva - start preparing for this now, by the way!) - you may receive some helpful feedback
- Focusing your research
- Good for the CV (especially if you get to publish)
- Usefull in making others aware of the research you're doing
- PG conferences are healthy introductions to the academic world, (usually) being more 'gentle'. Can therefore help build confidence
- AND useful for networking in other ways for the future
But on the downside
- be careful that you don't take too much time away from your PhD
- try not to ossify your ideas at this time: keep it as an open discussion, and don't be afraid to say that this is work in progress (your ideas will possibly change significantly over time)
Go for it if you can fit it in - but talk to your supervisor about it first (up)