Pet peeve time...
When a dancer, during a fast battement tendu combination at barre, allows her toes to lift off the floor and instead performs a battement degage.
Keep. The. Toes. On. The. Floor.
But most of all, it defeats the purpose of the exercise. I've blogged about the various elements of a barre in this post and what each is intended to do prior to coming to center, so I will just focus here on tendu.
The battement tendu, when performed slowly in either 1st or 5th position, helps warm up our feet and ankles and allows us time for proper articulation. We then take that articulation and apply it to a slightly faster combination with a new emphasis on stretch - we're still articulating the whole foot, ankle, and toes, however. Regardless of the speed, we still need to point the foot and engage all the muscles from ankle to toes. As the exercises get faster, it's even more crucial that we keep the toes on the floor because if we don't, we discard the work we just did.
Additionally, when you work really hard to keep the faster tendus on the floor, you will be engaging the inner thigh muscles, the external rotators and the glutes - all of which we need to help maintain our turnout in the center and perform fast allegro work. If you don't use those muscles but instead let the toes leave the floor, you end up using the quads.
And finally, it's simply not a clean line. When we perform degage, we are actively engaging our muscles to get the foot off the floor but when we do it during tendu, it's by accident. We are not training our legs and feet to complete the line so when we come to the center, the foot looks like a floppy fish on the end of a line.
Don't be a floppy fish.
Articulation. Strength. Line. Keep the toes on the floor.