The Acer has already been returned.
Really?! You returned it rather than installing another video player app (many of which are free)? No matter what tablet you buy there will always be formats/codecs that aren't supported by the stock player. Even the Archos stock player won't play AC3 audio or MPEG 2 video with buying their plugin.
This seems kind of like returning your PC because Microsoft Office or Photoshop isn't pre-installed.
Third party players only increase the supported containers. You can have the same h264 profile in MKV, AVI, MP4, but the stock android players usually only support certain profiles on MP4 containers. Rockplayer just lets you pull the data stream out of other file types and send it to the onboard decoder. If the hardware decoder doesn't support the data stream, then rockplayer plays it in software (Curiously, do dual cores now have the processing power to decode high profile HD videos in software?).
If it was a simple matter of running a third party program, you wouldn't have all those people asking how to re-encode their videos to run on the transformer.