There are, at present, 53 commons in the game (ignoring guardians). If I opened 10600 gold chests today, I'd get 5300 common troops, or "about" 100 of each.
Of those 53, I have 3 at epic, 3 at legendary, and 47 at mythic. So the 4700 that I have at mythic are wasted, and I get 600 useful common troops which would bring the 3 legendary to mythic and 3 epic to legendary. The extra kingdom power from raising those commons 1 level each isn't enough to upgrade a kingdom to the next star level. Then Suncrest comes out in a few weeks and there is a new common, one that I need in order to get Suncrest leveled up.
Instead, I'll wait and open 10800 gold chests when Suncrest launches getting 5400 troops, about 100 of each of the 54 commons. I'll ascend my 3 epic level commons to legendary and the 3 legendary to mythic. I'll also get the brand new common ascended all the way up to legendary! Net result is MUCH more value after release.
The same logic applies to rare troops from gold chests and glory chests.