As mysterious as derailures might seem, there is very little that can go wrong with them;

1) Bent hanger or other components
2) worn springs
3) worn chain guide wheels.
4) Worn pivot joint.

Worn chain guide wheels are easy and cheap to replace.
Worn springs and worn pivot joints are difficult enough to justify buying a new derailure.
Bent derailure components are tricky and not always fix-able; worth a try before tossing, but....

So, anyway, if your derailure is working, keep on using it. The "play" you are experiencing is most often in the shifters and cables rather than in the derailure.