What is the purpose of the crown in the middle of a router mounting plate?
I know the theoretical answer to this because I read it here, in reference to a different router table top: http://www.leevalley.com/en/Wood/page.aspx?p=41793&amp;cat=1,43053,43885
It's twofold, as suggested earlier in the comments:


it helps the table to sit flat, or at least not concave, with the weight of the router.
it ensures that the workpiece contacts the table right next to the bit, even if it is not quite touching out at the edges.


I also have seen this in practice, with my own makeshift router table top, made of too-thin plywood:


If the table sags, the accuracy goes horribly awry, because the depth of cut changes according to the workpiece's position on the table, as well as with the amount of sag (which tends to fluctuate with weight).
If the table is bowed up slightly, the error is not appreciable. Technique helps here too--you want to apply downward pressure on the workpiece near the bit, not near the edge of the table.


The key here is that the curvature here is very slight. The amount of tilt the workpiece might get with a convex table doesn't generally add up to appreciable error, but the depth variation with an equivalently sagging table is unacceptable for most joinery.

My experience suggests that the convex table is a good solution. Of course the other answer is to get something perfectly flat and sturdy, but I would be careful about that, because a lot of materials (like MDF) will sag over time.

EDIT: For a convex inset plate, install it so that it is flush with the rest of the table at the edges, and you should be in the same situation as if the whole table top was crowned.