A civil engineer makes 2 shields that are not subject to waste calculations. I can think of 2 situations where they're extremely useful.
The first is in those far-flung cities with maximal waste where you generate only 1 shield per turn. Irrigate the heck out of every tile so you can support a few specialists, then make a some civil engineers. You can fairly easily turn that 1 shield per turn city into a 5-10 shield per turn city. That's enough to make a temple, a barracks, walls, and/or other cheap improvements in a reasonable number of turns.
The second is in cases where your city needs 1 or 2 more shields to hit a nice, evenly divisible number. For example, suppose you're making 19 shields per turn. That means a factory takes 13 turns to build, a hospital takes 9, and a tank takes 6. Convert one guy to a civil engineer to get yourself up to 20 or 21 shields per turn, and now those things take 12, 8, and 5 turns to build. That may not sound like much, but if you're saving 1 turn every time you build something and in 5 different cities, that's a lot of free turns.