Depends on the birds you're shooting, if it's gliding/soaring you're fine on single shot. If it's powering through with lots of wing movement you soon appreciate what 10fps gives you over 4fps. I go out now with the 1Ds2 and get plenty of shots, none that I Iike due to boring or obstructive wing placement
FPS does help here but you also have to put things in perspective really, 10FPS vs 5FPS will mean you have twice the chances of getting a photo you want, which to a pro doing sports is life or death but is a tradeoff for amateur wildlife. Partly why I don't really see the big criticism of the D800 being 4FPS in FF mode.
The pixel density and reach are more important factors to me. To be able to produce similar sized prints of a distant animal/bird with a D700 I would need a 600mm f/4.0 but could get away with the 300mm f/2.8 or even 300mm f/4.0 on a D800. There is a big size, weight an cost penalty to get that pixel coverage. Of course I would prefer the extra reach to come from longer glass rather than smaller pixels being cropped but owning a 600mm lens is just not realistic.
The downside comes with less ideal light where small pixels will quicker become a downfall and the large fast glass on a lower pixel-density sensor
will really shine.
Still, BiF is probably the most demanding type of photography out there and high FPS certainly helps.
