If you can't see how being able to both build and repair (or build and decon) at the same time wouldn't significantly change the current balance of the game, especially during base attacks, there must be a problem with you. Perhaps it wouldn't be allowing a single builder to do the job of several, or even double his efficiency, but it would still dramatically increase his base repairing potential. Imagine the human base is under attack. A tyrant just took out two turrets before the humans could take him down, but there is also an advanced dragoon sniping the reactor. You're the only human builder. You now have a choice to make: start building new turrets to replace the fallen defenses, or run to try to save the reactor from barbs by repairing it. Under your suggestion, you'd be able to do both, removing the tension and strategy from the situation, as well as significantly boosting your defenses against the aliens. Now imagine instead of you being the only builder, there are two of you. Currently, one could start rebuilding the turrets while the other repairs the reactor. Under your suggestion? Both turrets can begin rebuilding immediately, AND the reactor will get repaired TWICE as fast!
Besides, marked decon makes rapid decons easy and quick, so your point is moot.
