The Bad

OK... having covered the advantages, now for the disadvantages.

The big disadvantage: clothing. Or to give it its proper emphasis, CLOTHING.

After all the drawbacks of clothing-as-texture I cited, why is this a disadvantage?

Prim breasts are attachments, and SL assumes that all attachments are meant to go on top of clothing. That's fine for jewelry, but not so good for body parts. So far, prim breast makers haven't used textures for clothing; instead they build them from prims. Such clothing and breasts have to be built together in one attachment, because there can be at most one attachment at an attachment point. Thus if you want to be clothed sometimes and not clothed other times, you have three choices:

1. Do without them when undressed.
2. Have two versions of the breasts, one clothed and one not.
3. Script the breasts so that with a command the clothes can be made transparent. At least one breast maker does this.
   

UPDATE: Oh, yes... because prim breast clothing is made of prims rather than painted on, it shows up problems with SL's rendering:

• SL has a way of throwing away details as you move away from things. For tops built from prims, this has the startling effect of making holes appear in them when viewed from afar.
• Tops and breasts made in the pre-sculpted prim days have to be pieced together to get the desired shape. The places where the pieces join are just about impossible to make smooth, i.e. there's a slight corner where they meet. SL lighting makes this corner stand out. I would love to see Foxbean Laboratories' "Marie" model with laced top remade with sculpted prims to avoid this problem.
  

Because prim breasts are unique; clothing that works for one pair doesn't work for another. Prim breast makers must therefore divide their attention between the breasts and clothing, and clothing necessarily gets short shrift in comparison with standard clothing, which has a multitude of sources and levels of quality from basic to exquisitely detailed.

The mainstream clothing designers need "only" create one texture for a top, and it will work, more or less, on any standard avatar, no matter how the sliders are set. (Only in quotes because doing it well is not an easy task!) They understandably don't consider it worth their while to create over a dozen different versions for a relatively small market.

You should be sure of exactly what you're buying. If your prim breasts don't come with a specially-made top, you have three choices:

1. Go topless.
2. Commission a top.
3. Build one yourself.

The first is a bit impractical in most places. The second will be quite expensive, being custom work. The third is good if you have the time and skills involved.

All this means prim breast wearers are cut off from the vast majority of the cornucopia of mainstream SL fashion, because many tops won't match up with the specially-made tops. Tops typically require adaptation or editing to match up reasonably with those standard tops they can be made to work with. Sometimes you can't tell whether the standard top will work without putting it on--but by then, you've already bought it.

Other things you should keep in mind:

• SL treats all attachments the same way. If, as most residents do, you wear prim hair, you've had the experience of arriving bald and waiting for your hair to rez. Prim breasts and their tops are the same, so you'd bettter have on a standard top to avoid major embarrassment.
• You've also probably had the experience of attachments appearing between your legs when you arrive... yes, it happens for prim breasts, too.
  
• Adapting prim breast tops to standard tops requires modify permission--and as a practical matter, copy permission unless you like doing the same customizations over and over again and are perfect, never making mistakes you can't undo. Be sure to check the permissions on what you buy.
• Poses and animations assume particular body proportions, and prim breasts are pretty sure to violate those assumptions.
  
• Clothing isn't all you have to adapt prim breasts to; there's also your skin color.

P.S. JIRA entry VWR-1064 asks that attachments have an attribute to indicate whether clothing should be drawn on top of them.