Then there's a lot happening. Those ~ tokens behind the execute command? They're used to indicate the location from where the command should be executed. And then we have the same thing behind the summon command. So, my take on this is that the /execute command looks up a location (which isn't even used) after which the /summon gets executed which also looks up a location (2 blocks higher than the players on team mojang).
That's 1 totally unused lookup, and lookups will gobble up resources (cycles), even if it might be just a little.
I doubt you'll notice much when using 2 or 6 commands, but now imagine a redstone contraption which has 75 of these /execute commands. And let's say that 60 of them don't even use the ~ tokens, they're all as above: ~ ~ ~. Waste of resources.
So: /as @a[team=mojang] summon minecraft:zombie ~ ~2 ~
Only 1 lookup is being done, and that lookup is also actually being used.
Now, don't get me wrong here: I do agree with you. Changing this stuff somewhere half way seems very useless, and annoying too. And some of those changes seem kind of dumb... /gamemode 1 no longer works, it now has to be /gamemode creative. So, on my server: /gm 1 no workie: /gm cr<tab>. More typing
But others, like the example above... I think there's a good reason behind it.