I personally prefer strong warriors over weak snakes and wyverns. Maybe they're okay at the beginning, but later on, their speed doesn't matter much when you're attacking hordes of strong units or a large army.
Firstly, they slow down the army. At the beginning of the game, you just have a "tank" unit following you, but later on, a situation might arise where the main hero has control, but the tank doesn't. As a result, you can't regularly transfer the army, and it can cause slowdowns.
Also, the enemy can emerge from the fog and catch you off guard without an army.
Secondly, even the strongest dwarves and gorgons will inevitably die in the later stages. In this regard, only tanking with a 6th or 7th-level unit will be valuable. And while a 6th-level unit can still slow down the enemy, there are very few low-speed 7th-level creatures, and even those aren't completely useless (giants have a decent speed, for example).
Thirdly, high-speed units are simply _essential_ in some quantity. They're the first to cast spells!
All these units can only be valuable in the endgame... But that's only if you don't get replacements for them from various sources like treasure chests and secondary cities. Otherwise, you just slowly sacrifice them as cannon fodder.