I think this is a good question whose benefits seem obvious but we should also consider the drawbacks related to adding this feature.
Existing tools can handle this based on browser and even operating systems. I've setup my Mac OS X for example, to speak highlighted text in Mandarin, English, or Cantonese (using keyboard shortcuts). Other tools that I have setup for the Mac include convert selected text to pinyin and lookup words in Chinese-English dictionary.
In fact, one interesting tool that I find useful is to collect phrases and store them in the Google Translate Phrasebook. Using that tool one can also capture the valid pinyin and save resulting phrases across many devices. Phrasebooks can also be saved as cvs files so offline uses are possible.
Sometimes the benefits of a process outweigh the drawbacks and in those cases one should take action. The question becomes is this benefit significant enough to outweigh the drawbacks of the effort to design, implement, and maintain a viable solution. If not then action should be constrained. From my perspective, given my existing toolset and other options, I cannot see a demand to proceed at this time. I am however open to change if a sufficient argument can be made to proceed with this option or even with a different existing tool.