It is being used as part of a combined system. For example AWS is pushing for Trainium + WSE 3. The WSE 3 does the decode and the Trainium does the prefill.

Even in nvidia land rubin + LPU does a similar thing.

It has its downsides of course - if your traffic swings prefill heavy to decode heavy, you can't suddenly use your lpu for prefill. With GPUs they're totally interchangeable. Tradeoffs.

AFAIK You can use WSE/LPU for prefill, it's just less efficient to do so.