Let's compare speeds, PapaLars? ;)
Crossbowman: 7
Angel: 10
Champion: 6
Thunderbird: 6
Naga: 7
Dark Knight: 9
Titan: 6
Vampire: 8
Venom: 5
Crystal Dragon: 5
Devil: 7
Ifrit: 6
Unicorn: 6
Black Dragon: 7
Phoenix: 12

In total, Champions potentially outpace Thunderbirds, Titans, Ifrits, and Unicorns. And they definitely outpace Venom and Crystal Dragon. The rest will almost always be faster. Do you think there's someone who would let you attack with 20 Champions at full speed, when they have faster units? Also, don't forget that with such low speed, Champions aren't very dangerous in a wait state. A speed of 11 (with a Tactician) can be easily outrun by a hero with a couple of crystals and one speed-boosting item. And even less than that. Such conditions exist on every map.

Yes. And one more thing. The first row usually consists of weak units. And after that, Champions can't break through. That's why they only play a role when there's either a huge number of them or they're in a "dog-eat-dog" situation. :).

That's the situation.