The round coolers remain to be popular with those who fly sparge because (1) they readily accept off-the-shelf false bottoms, rotating sparge arms, etc., (2) they don't have corners at the bottom, which makes for more even lautering (especially with a false bottom), and (3) they are tall and cylindrical, which makes for a deep grain bed.
These benefits are generally of much less importance to batch spargers. The only (minor) benefits are that I find that the round coolers are a wee bit easier to stir, because there are no corners (possibly offset if you have a big, deep grainbed, however), and they are consistently well insulated (you have to shop carefully for well-insulated rectangular coolers as some are pathetic).
One big advantage of rectangular coolers is that they are the only ones readily available above 10 gals in volume. They also come in a wider range of sizes and shapes than round ones.