Lasers Brightness vs Armor Effectiveness
The US military has abandoned a number of research projects over the years into the use of plasma and laser based small arms because of things like power sources and maximum energy outputs, but in recent years, those problems have mostly been solved here in the real world. So why are we not seeing our armed forces equipped with laser weapons yet? Well one of the biggest remaining hurdles is that they reflect and scatter so much light that they blind anyone standing near by. So, if you were to arm a squad with laser rifles, every time you shoot someone, everyone in the area would be blinded... this is especially bad for the people using the laser rifles because most of the people on the battlefield looking at the flash of light when it happens would be friendly forces leaving your whole squad vulnerable every time you go to shoot someone.
If you think of it like this, the alibeto of your average military uniform is about 0.3 meaning that 30% of the light that hits it is reflected. The lowest power laser theoretically capable of burning a person faster than they can get out of the way is ~1kw. This means that you are looking at a spontaneous flash of light equivalent to a 300w flash bulb. To put this in perspective, the flash bulbs used by most professional photography cameras are about 70-400w. So, at the very low end of anti-personnel lasers, the flashback would be momentarily blinding, shooting up an enemy position would be like having a line of paparazzi flashing cameras back in your face... but this is just for weapons of the power level required mow down unarmed civilians.
If the development of laser technology stopped here, then you could just have your soldiers put on some goggles that filter out the wavelength of the lasers and then they could go wreak some havoc by blinding and disorienting their enemies while they shoot them up.
The problem comes in when armor and weapon developers start trying to one up each other.
Whenever you introduce a ubiquitous weapon technology into a setting you need to assume that the opposition is already aware of it and would try to find a way to counter it. This brings me to what anti-laser armor would look like. Through the use of dielectric mirror based armor, enemy tanks, aircraft, and soldiers could adapt by using anti-laser armor that reflects at ~99.9999% of a laser, as long as they know the wavelength the laser is operating at. This means that laser designers would have to design their weapons around firing a lot of converging lasers of different frequencies (which they already should be doing anyway to reduce thermal blooming).
That said, dielectric mirror are no good against multi-spectral attacks. The most reflective known broadspectrum materials can only reflect 99.9% of light, but as comments point out, even this is much more ideal than you can expect under battlefield conditions. In terms of practical reflective armors, about 80-90% is probably the best you can expect under combat conditions... but this is still enough to be a problem. This means that if enemy soldiers simply start to use bright white uniforms, they could make themselves pretty hard to kill at just 1kw. So, HEL designers would need to scale up to 3.5-7kw to burn heat resistant white clothing. but in doing so, you increase the total reflected light by somewhere between 933-2100%. At these intensities, the reflected light would be enough to risk long term damage to the eyes or anyone not wearing proper protective gear including innocent bystanders.
But reflective armor is just one way to increase resistance to lasers. There are other technologies like thermal ablative insulation which can make it incredibly hard to burn through something with a laser. The more you layer known defensive technologies, the more powerful lasers will need to become to cut through them.
By the time laser armor starts to mature, you could be looking at needing laser rifles hundreds of times as powerful as those basic 1kw lasers if you want to actually do any damage. If we pretend that portable laser technology could be escalated to meet the challenge, you will now be looking at amounts of reflected light equivalent to thousands of high end camera flashes going off in your face across a broad spectrum of light frequencies. As nick012000 pointed out in comments, this is VERY bad for any innocent civilians anywhere near the battlefield who happens to not be wearing some very good protective eye gear. Furthermore, because it is a broad spectrum reflection, you can no longer use simple filtered visors anymore. Your only real choices here are blackout goggles that momentarily shut out all light when a laser hits something (blinding you but not permanently), or to have a closed faced helmet that uses a camera and internal display that does not get bright enough to ever hurt your eyes, even in white out situations.
Any way you add it up, using HELs able to deal with armored opponents would simply light up the battlefield too much to actually have a battle, and the collateral to civilians would make the weapons pointless for 95% of operations where ground troops would otherwise be the preferred tactic of choice.
So why use M/DEW-96C Laser Cannons?
For the M/DEW-96C Laser Cannon, this is less of an issue. Although it is a higher energy laser (probably in the 10s-100s megawatt range if you want to kill laser armored vehicles), how you use it is different. Whenever you need to blow up a tank or a building, your whole squad other than the person attacking already has to take cover so they don't get taken out by debris. So everyone in your squad also having to shield their eyes as the one guy takes the shot is a reasonable additional action. And for the person attacking, they only need to make one shot; so, they could have some kind of blackout goggles so that your Laser Cannon Troopers eyes don't get seared out of his face, and not be left at a huge disadvantage not being able to make an immediate follow up shot like you would expect to want form an anti-personal gun. Also, when you need to blow up a tank, innocent bystanders are generally already much less of a concern than when performing more surgical infantry operations.