It really depends on what you are trying to brew.
Dextrose adds no flavor, no color, no body, no head (whereas extracts will add to all of them) - it ferments pretty much completely and does little other than add to your ABV. It is usually used in recipes as a substitute for grain or extract when the beer would end up too dark or with more body than desired (i.e. you want a lower FG). The process of making malt extracts (dry or liquid) involves an evaporative step that essentially cooks the wort for you (100% extract recipes really don't require boiling to work, except for the matter of hopping). Like boiling your wort, this darkens the extract.
Back when the Coopers+sugar kits appeared in the late 60s/early 70s, people wanted to brew light beers similar to Ballantine, Stroh's, PBR, and Schlitz (basically the SLIGHTLY more flavorful but very light colored forerunners of Coors, Miller, and Bud). This was also still a time of American pride (Vietnam not withstanding) and "all-American" homogenous products like Wonder Bread were a source of both comfort and national pride - people simply didn't want abeer that looked or tasted European. Thus, lighter color and lighter malt flavors were desired in homebrewing and using a large proportion (often 50%!!!!) of sugar (of course, it was usually table sugar then) was considered the best way to achieve this.
Modern tastes are quite different and homebrewers revel in all sort of dark and flavorful beers - and knowledge of the details of brewing has advanced by leaps and bounds. Achieving lighter color with extract recipes is currently done by adding a little extract at the start of the boil to aid in hopping and the bulk is added close to flameout (i.e. end of boil). This allows the extract to avoid further darkening in the boil, yet retain the full flavor of the malts (and avoid the classic "cidery" taste from the old sugar-heavy recipes).
Personally, I only add dextrose in a few situations:
1 - If the OG is correct and the FG is higher than I want, I'll substitute some dextrose for grain/lme.
2 - If my OG is too low at the end of my brew day, I may boil some quickly and add it to the chilling wort.
3 - High gravity brews benefit from extra sugars being stepped in during fermentation to ensure the complex sugars get fermented first.
4 - Priming for bottling (although I use honey, maple syrup, or extract almost as often - I've even primed with molasses before).
5 - If mid-brew on a day the LHBS is closed and I'm short on a grain or extract with no good substitute, I MAY use dextrose (but also may wait until the next day and do a mini-mash and mini-boil to add it to the fermentor).
6 - Wines and ciders = totally different ballgame. Dextrose plays better with fruit sugars than grain sugars. (Hops also play a role in the development of the "cidery" off flavors.)
Sooo...............whatcha brewin'?
