What purpose is the firearm that you're going to be carrying used for? Is it for duty, personal defense, home defense, or range use? That's going to be the first factor in determining if a weapon mounted light (WML) is going to be worthwhile.
For a duty gun, it's absolutely worth having a WML. Duty gnus have a very real possibility of having to get used in an offensive capacity, where you are actively searching for a hostile subject. Your gun should be out, and you will need a light to positively ID whether or not the target you have in your sights is a threat or not.
For a personal defense or home defense gun, it is beneficial. No, you will not likely be actively searching for a hostile subject, but you will still need to PID the target. This could be a robber that's approaching, and you want to get better lighting on the threat. This could be the random person that jumped out and presented themselves as a serious threat. This could be the person banging on your bedroom door trying to get in, and you want to clearly see where they are when they do break down the door.
As a range gun, it's not necessary, unless you plan to shoot a lot at night.
Why a WML over a handheld light? Firstly, the two serve two different primary tasks. A handheld light should be part of everyone's EDC kit, and it allows you to illuminate an area for everything else that doesn't require a lethal response. The secondary (and equally important) purpose for a handheld light is to provide illumination on a potential threat, and maintaining illumination while engaging said threat if it becomes a lethal threat.
A WML's primary purpose is to provide PID on a lethal threat. It is NOT purpose built for lighting up an alleyway, or finding your keys, or providing lighting on an approaching unknown person. A WML is used only when that person or thing becomes a lethal threat, and you want to use a two handed grip on your gun while maintaining illumination.
So if a handheld light can provide illumination, why use a WML? Because a WML allows the shooter to use a two handed grip while illuminating a target. A two handed grip is much more stable than any one handed grip with a light, which then allows increased accuracy, faster follow up shots, and better overall effectiveness. This translates to better hits on target, more hits on target, and being able to see if there are other threats after the initial target is down.
Now, if there are other potential threats, then you can continue to use your WML to scan and assess, and then engage if necessary. For duty use, this is an easy concept. For defensive purposes, this may or may not be necessary. However, it is still highly useful because it allows you to maintain a two handed grip, maintain illumination, and keep your area secured until law enforcement arrives.
There are a number of techniques available for use of a handheld light and WML. This is where classes and genuine instruction comes into play. You'll be able to learn how to properly use these techniques, when they are most useful, and then be able to practice what you learned. You can still learn on your own, but it's always better to see what the proper and actual application is, rather than try to do it yourself with zero experience, since this can often lead to bad habits. Again, you CAN learn on your own, but it tends to work better with genuine instruction.
IF you choose to run a WML, then you need to consider how much light you think you'll need, what size light you can practically use, cost of the WML, and cost of any accessories you need to purchase (holsters and batteries). I have half a dozen holsters that were rendered useless when I picked up my WML for duty and EDC. The cost of new holsters added up quickly.
As for how many lumens and candela? I prefer as much as possible. Yes, there is a very real possibility of light backsplashing and blinding you. This is where proper training and practice comes into play. At a minimum, I would say 300 lumens, but I would rather have a 1,000 lumen handheld and an 800 lumen WML. The more light I have, the more information I can gain from the light used, and the better my chances of completely blinding a potential threat.
Durability also needs to be accounted for. A WML is going to take a lot more abuse than a handheld light, by sheer virtue of the nature of recoil from a handgun. That's a lot of heavy jarring on an electronic system, and durability is going to be a must. Purpose built WML's will withstand this better than a cheaply made airsoft model. My go-to brands are Streamlight and Surefire. I give a heavy nod to Streamlight, because their products have been proven to be more than durable, and their price points are half of what Surefire charges.
This only scratches the surface of this topic, and there's a lot to consider. I think it covers a lot of the basics though, and if anyone else has anything to add, feel free.
ETA: my personal setup is a G22 w/ Surefure X300 for duty, and a G23 w/ Streamlight TLR-1HL for off-duty. I'll be switching to a TLR-1HL for duty use ASAP.