Nets can familiarise bad habits as well as good. Sloppy/lazy technique when batting and my pet hate, bowlers no balling.
You need to have an objective, not just aimlessly bowling or aimlessly having a whack.
I try to work on specific weak areas of my game over the winter with the bowling machine (this year it's the short ball) and any club nets with real bowlers I treat like an innings, ie I value my wicket, I leave the ball, I punish the bad ball but I don't try to murder everything.
As the season approaches and then in the season itself I tend to just use the machine to keep my eye in rather than work on anything in particular, unless I identify a flaw that I've either not ironed out in the winter or have developed in the summer. Over thinking/analyzing the mechanics of batting (unless something is seriously wrong) usually just causes more issues.
Bowling wise I can't help much as I'm a gloveman. But please don't no ball, or bring your special shiny ball that hoops round corners! I'm not using a wider bat or playing in front of smaller stumps!

In short, I don't think they are required, they are a luxury. I didn't net regularly before vitas cricket existed, and didn't suffer too badly for it. However from a team perspective, I think everyone needs to train together.
From a personal viewpoint, unless you go into a net with a specific objective (ie don't carelessly no ball or aimlessly dig everything in short, and also don't try to cream everything with the bat) netting can do more harm than good, by encouraging bad habits and endorsing lazy technique etc.