It's nice to have numerous arrows in your quiver. When your legs are cramping from one type of kick, it's nice to able to switch to another kick.
I'm not a big fan of the frog. It's not really an efficient kick, as far a gas consumption is concerned, and the frog takes bit of practice to get it down efficiently.
The modified frog cannot be matched when it comes to precise maneuvering in tight spots.
The flutter cannot be matched for power, speed and endurance.
I have done some gas-consumption studies using a variety of speeds and kicks. Given that drag in water increases by a power of 4 with increasing speed, it is clear that the faster you go (regardless of the type of kick) the more gas you consume. I found that, for any given speed, I use the least amount of gas per distance traveled by using a lazy flutter kick.
As mentioned above, a good modified flutter is very efficient and has good anti-silting potential when done correctly.
The porpoise kick is really fun and is nice for a switch when you want to use a different set of muscles, but it's got to be used only in wide-open spaces since it's so... vigorous.
It's also nice to practice one-legged kicks, in case you ever find yourself in a situation when you've lost a fin or injured a leg.
Sculling-type kicks are useful for maintaining a steady position (you can scull forwards or backwards as needed to hold steady), and for helicopter turns.
And then there's the backwards kick, useful for maintaining position and stopping quickly. A must kick for photographers.