I use troops all the time. I rarely decrease my ground combat level, but I never raise it either. When almost everybody is dropping ther's to 50, 100 does just fine.
Glassing your opponents planets is like pushing a ball down a hill, capturing them is like pushing a ball off a cliff. It's speeds up the process.
If it's a border system that I can't defend effectively, I like capturing the world, loading as much of the pop as I can on my transport, scrapping the fac's and leaving the planet for the enemy to glass. Put the pop that's left to work building some mines/sats/fighters, just to make it a little more trouble for your enemy. Works even better if you can get the planet that has the space port.
Geoschmo