A terran player can not go both air and mech effectivly at the same time early on. If he is air he has to upgrade air, and if mech he has to upgrade mech. Upgrading both, especially at the same time, will result in a huge loss of resources which would allow a zerg player a window to attack cause the terran player is having to hold back troop production for the techs.
You could look at it as the first 2 upgrades = 1-2 units, next 2 are 2-3 units and the final are atleast 3-4 units worth so upgrading them all sets the terran back over 6 units per branch. Going both mech and air is a setback of 12 units worth.
Although the yield from the techs makes up for it or the cost does not mater at 200/200 it does set you back early game cause if you stayed in only one branch you would have more troops. This is not counting the fact that you will need to produce 2-4 starports with reactors or techlabs which is another 4 or more troops behind.
What is important is you have enough Air to air potential (vikings) to discourage the enemy from constructing broodlords if you are going mech however built vikings are totally usless against ground units so you are making yourself more at risk to zerg land forces (ultralisks, speedlings etc). It comes down to a battle of wits to make sure that you only can counter if present to counter.