Two reasons I tend not to recommend it to clients.
1 - It is significantly harder to reinforce your dog for seeing other dogs - necessary to alter the emotional response that is behind the reactivity - if it is difficult to get a reward to the dog. Now muzzles can be adapted, but it still takes a lot longer and speed is of the essence, you need the reward to be delivered within 2 seconds. If the dog won't take food as a reward but prefers tugging a toy, catching a ball (Which with a ball on a rope can be done on a short lead still) then the muzzle means no reward possible.
2 - It actually can mean people are MORE likely to let their dog run up close to the other dog, because they know that dog can't do any harm. At the same time, it also tends to cause owners to put their dog in situations the dog can't handle, because they have that false sense of security the muzzle offers. Yes, the muzzled dog can't bite, but the muzzled dog practices unwanted behaviour, stress levels go up, fearful situations are repeated... the problem gets worse.
In some situations its necessary - if you can't get out of your house without the risk of encountering people/dogs, whatever the dog reacts to, then you may need to muzzle til you get where you're going.
If the dog has a bite history, then obviously it is necessary - reactive does not = bites.
In some situations you can easily reinforce with squeezey cheese, people in that particular area stay away from muzzled dogs rather than let dogs run up to them... so it might be more appropriate.
For my clients (I obviously cannot speak for all owners) - by the time they are walking their reactive dog where other people may be walking theirs, that dog has gone through a ton of training and is already well on the way to being 'fixed'. We'll have worked in quieter environments, built value in seeing stooge dogs and disengaging and looking at the owner for reinforcement and further instruction, gradually worked up to walking in the places the owner would prefer to walk.
However it will still only take one muppet to let their 'it's ok hes friendly' dog come barrelling over to within a foot or two, to upset the apple cart and set that dog and owner back a long way.
If your dog does not automatically check in with you on seeing another dog - on or off lead - don't have your dog off lead! It is not enough to recall your dog after they've already got within a few feet of a leashed dog and upset it, they need to stay a decent distance away - 10ft would be good, 20ft would be much better.
You may think you'll never end up with a reactive dog, but unless you are extremely lucky there is a good chance you will. So much depends on other people being responsible, on top of you doing the training and being responsible yourself.
ETA, Pissflaps, I can't edit out the quote.