The idea behind a fly sparge is that the sparge water sits on top of the grains and wort. As you drain (and add more water), it pushes the sweeter wort down and out and picks up more sugars as it passes through the grains, becoming ever weaker wort as you add water on top and it drains through. When you stop draining, you're typically leaving very low gravity wort in the MLT.
With a batch sparge, you're mixing all the sparge water together into one homogeneous mixture. Therefore, whatever you leave behind will likely hold more sugars than what you'd have with a fly sparge. As noted above, this will be a hit to your efficiency.
I calculate my mash and sparge volumes for a batch sparge to get as equal a runnings as possible. Experiments show that the closer to equal they are, the better the efficiency.
In general, efficiency isn't a big deal as you can add more grain to compensate. Where it can become a problem is with very high gravity beers. The reason is that, as the grain bill goes up (for any given volume of wort into the boil kettle) efficiency goes down. What can happen is that you add more grain, but this drops you efficiency, so you add even more grain, which further drops your efficiency (etc. etc.) and you can get to a point where you're just spinning your wheels.
If you want to brew high gravity, it's worth keeping track of your efficiency. If it's around 70 or higher (brewhouse) with a 1.060 OG beer, you'll be fine with the higher gravity beers. If it's down near 60 or lower, you'll probably have a hard time.
I use brew365.com to calculate my water. After you figure out your equipment (grain absorption, dead space, etc.), you'll be able to nail the strike volume and temp along with the sparge volume.
