I think plants are chemical like self assembling factories so they have potential to be used like factories today, but it's probably not worth the risk of unpredictable interactions in the biosphere which seem far more likely if you engineering biological factories vs mechanical ones.

It's good imagination fuel and while plants are best at making other biological products their is some overlap .. like plants can make fuel even though we don't view it in the same category as food and medicine which they also make, but if we tried to make flat screen TVs with plants I think their rate of production vs land use would not be worth it.

I think self assembling robots will generally be better than involving biology unnecessarily, we have a lot more control and less chance of side effects in out giant soup of a biosphere.