"pointed out that you might as well put the first trumpet up there for the first movement too"
No, because a trumpet is primarily a cylindrical bore instrument, pitched high, and with the bell pointing forwards. Trumpets can cut through anything! Case in point: The Chicago Symphony, where Orchestra Hall is relatively shallow, so the trumpets pretty much dominate over everything. In my opinion, it would probably sound awful to have the solo trumpet fanfares played up front.
The business of having the solo horn up front has a lot to do with the acoustics of the hall, as well as the particular soloist AND the particular type of horn they're using. For whatever reason, the Concertgebouw hall is very helpful to woodwind players (it's the steep tiers), but not too generous to horn players. Having the horn soloist up front keeps them from having to blow hard the entire time. In the Vienna Phil., they use the indigenous Viennese single F-horns, which have a pretty huge sound regardless of how soft or hard you blow into them. Because they're the closest thing to a valveless "natural" horn, the tone of the Viennese F-horn just carries at any dynamic level. In addition, Wien's little Musikverein is very generous to the horn section. The situation with the Berlin Phil. is almost the exact opposite. The Philharmonie is not real helpful to the horn section (I saw Barenboim do Bruckner 9 there, and had lots of trouble hearing the horns clearly), and they tend to cling to their relatively small Alexander double horns. Alex.'s tend to get a little "bracky" sounding when you really lean on them. Therefore, in the case of the Rattle DVD, I don't think it hurts to have the solo horn up front at all. To my ears, the Chailly/Concertgebouw sounds pretty darn good as well. Comparing horns to trumpets is truly just like comparing apples to oranges.