I've been involved in it both ways,
I worked for one Firm and we did all of our visualization with in the office. But we had about 50 poeple in firm so I guess, our 2 salaries for the Visualization team could be absorbed into marketing budget.
Now I work for a small firm and we contracted out a rendering/movie. Personally I think it was a diseaster. But I blame the person who hired the visualization firm. The product was junk and the useless communication back and forth cost our firm way to much time/money.
If you can afford to have top notch in-house or you want to bounce around renderings during schematic design do it in-house. Otherwise have clear goal of what product you want and contract out the best you can afford. If you go cheap you pay in the long term trying to make the donkey run like a horse. Architects are just to picky...