I first joined a gym when I was 17, back in year 12 (I'm glad I did...it stopped me from stressing too much in that HSC year!). I had a personal trainer who showed me how to correctly execute a number of exercises. In hindsight, that was a really smart move.
Because I trained by myself and just for general physique, I can't say that I was really into the "weight game". But that changed somewhat a year later when I joined a more popular gym and started training with some friends. My training rationale would go something along the lines of "the more weight the better!" But you quickly learn.
I was fortunate in that I didn't sustain any major injuries. Admittedly, I wasn't completely negligent in my approach, but not nearly as careful as what I am now. Two reasons for this:
1. Throwing around weights at the gym can be really dangerous.
2. Through the theory learnt when undertaking studies for my qualifications, I learnt that sacrificing form for more weight didn't necessarily overload that muscle more.
Whenever I train a client, I place a heavy focus on technique. I am a strong believer that proper form is the foundation to great results in the gym. Ultimately, if you are trying to overload a muscle - overload it with good technique. As soon as you begin utilising other body parts, you are only recruiting other body parts - that muscle being targeted isn't necessarily being overloaded any more effectively!
Here's a classic example - standing bicep curls. Textbook form is to keep your back straight and curling up without your upper arms moving forward. What I consider "sloppy" form is when you recruit your legs and back in a swinging motion. Consider what is actually happening here:
Your biceps are only so strong. They can only generate so much force. By swinging, yes, you can move more weight...but your biceps aren't doing the additional work to lift the bar up.
Unless you are an experienced weight trainer and have a good degree of body awareness, you are significantly increasing the risk of injury.
Ego can play a real part in the temptation to "lift as much weight as possible". I would guess that the majority of weight trainers have played this game at one point in their life. But seriously, who cares if the person next to you can lift more weight? Personally, I'm quite happy competing against myself trying to lift more than the previous week. I honestly find it silly sacrificing technique in order to impress someone else.
I will admit that there are more advanced training techniques whereby it may be necessary to move away from "textbook" form. A good example of this may be if you are trying to overload the eccentric phase to each rep. Since this is a more advanced way of training, it really isn't recommended for inexperienced weight trainers.
Of the inexperienced weight trainers that I see in the gym, more than half of them don't know how to correctly execute various exercises in a safe and effective manner.
So there are my thoughts on poor technique. Unless it's for a specific purpose, take a look at your form and fix it up if you need to.