OG/SG and efficiency readings are really only REQUIRED if you want to EXACTLY duplicate the beer at a later date. FG readings are really only necessary if you want to know exactly when the beer is done fermenting.
If you like to make a variety of beers, don't care if your next batch of XXX Ale is exactly like the last batch, as long as it's in the ball park, and give your beers plenty of time to ferment out, then taking strict gravity readings are certainly not particularly important.
I'd say a good # of homebrewers fall into this category, but there are definitely just as many that like to control and understand every aspect like a laboratory experiment.
The ones with a more controlled process probably make slightly better beers on average than brewers who wing it, but the wingers still make great beer as well!