Some people think that druids can use wild shape to enter the form of a swarm. But isn't the point of the Swarm Monger Druid archetype to be able to do that? Since an option exists to grant that ability, doesn't that mean that regular druids must be unable to do so? (Inspired by this comment by ShadowKras)
The Swarm Monger is all about vermin
(and less about swarms)
The Swarm Monger is special in that "Whereas other druids commune with nature or even the spirit of a city, swarm mongers find beauty and strength in decay, and they draw their power from fungus, disease, and their own singular will to survive." (swarm monger archetype description). While vermin in Pathfinder are often encountered in swarms, they can also be encountered individually (as many of the swarm monger's abilities attest), and it is the entirety of the vermin type, not 'swarms', that the swarm monger is truly at home with, both mechanically and thematically. In fact, swarm-based wildshaping isn't even available to the swarm monger until 12th level. Instead, the swarm monger gets a magically hybrid individual/swarm familiar/animal companion, which is pretty cool.
The swarm monger is a fairly interesting and moderately well-designed druid archetype, with several thematic abilities that nonetheless leave it roughly equal to the base druid in overall power, since druid power comes from spellcasting which is unchanged. However, the swarm monger is not all about the ability to wild shape into swarms but rather about druids communing with the decompository aspects of nature as a whole, in particular through the 'vermin' creature type as emblematic of decay. This is good, because if it was all about shaping into swarms, most of its abilities would have been useless (as we will see below).
Options that let you do things don't mean that you couldn't already do those things
Pathfinder, unfortunately, is sometimes badly written. Whether through editorial negligence, a lack of playtesting, authorial incompetence, or some combination of all three, mistakes are made causing material to be published that should really not have been published that way. That's what errata's for, and sometimes errata fixes the problem, but other times things are just left that way and, if the material is used in-game at all, groups will have to figure out what to do with it.
One kind of poorly written material is the 'trap' option. A trap option looks good, and has exciting fluff, but does too little/nothing/is actively harmful to the character who takes it to use in its apparently intended purpose. 'too little' options include things like the monk class, and 'actively harmful' include things like Monkey Lunge.
'Nothing' options are most commonly options that give you abilities you could already do. These are particularly problematic because they imply that you probably can't do the things they claim to let you do without them, and may thus cause unwitting GMs to erroneously rule that way. In this way they are a trap option not just for the player, but for the GM as well. If Swarm Monger did have a whole bunch of exceptions allowing you to shape into a swarm (which it fortunately does not!) it would be one of these options, and best avoided. While options may imply that you can't do what they give you without them, there are no rules to that effect and, in fact, case by case analysis is needed to determine what an option gives you beyond your preexisting abilities.
Druids can shape into swarms, but only a couple
The wild shape ability states, in part:
At 4th level, a druid gains the ability to turn herself into any small or Medium animal and back again once per day. Her options for new forms include all creatures with the animal type. This ability functions like the beast shape I spell, except as noted here. The effect lasts for 1 hour per druid level, or until she changes back. Changing form (to animal or back) is a standard action and doesn’t provoke an attack of opportunity. The form chosen must be that of an animal the druid is familiar with.
Swarm is a subtype, not a type, and there are a number of animal-type creatures with the swarm subtype, such as the rat swarm and bat swarm. Since an [animal] swarm is a creature with the animal type, a druid familiar with a given [animal] swarm can wild shape into one starting at 4th level assuming they meet the rest of the requirements. In actuality, however, they will not be able to shape into any swarms (barring arbitrary-size-changing house rules, like those used in the linked question) until they can shape into tiny animals (6th level), since no published animal swarms are bigger than tiny.
Remember, also, that the ability functions like the beast shape line when an animal is chosen, and that beast shape I states:
When you cast this spell, you can assume the form of any Small or Medium creature of the animal type.
and beast shape II increases this to:
This spell functions as beast shape I, except that it also allows you to assume the form of a Tiny or Large creature of the animal type.
Which includes swarms. However, turning into a swarm, much like turning into anything in pathfinder (short of PaO) doesn't actually grant you the important parts of what you are turning into, most of the time. Remember that when you wild shape, most things stay the same.