It is true that there are some slight weight savings available with the various pressfit bb designs (exact weight savings obviously vary depending on system, frame manufacturing techniques, and crank model), but we don't feel this small savings make up for the inconveniences. We are still able to make a frame that is lighter than most of our competitors, while still using a heavier bb system. There are a number of disadvantages that exist with press fit systems:
1) Special installation and removal tools are required for these parts, including a headset press. This is not convenient for most home mechanics, and they are quite expensive. Traditional external BB's can be installed or removed with a simple $10 hand tool.
2) "Permanently installed cups". Shimano doesn't recommend removing and re-installing their press in bb cups (as they may become damaged), so moving parts from bike to bike is no longer an option. http://techdocs.shimano.com/media/techdocs/content/cycle/SI/SI_0053A_001...
3) Creaking or shifting bb's can be common with these systems. Since the bearing is pressed into a cup, which is then pressed into the frame- it can be hard to get all of the press fits snug- without being too tight on the bearing or too loose in the frame.
4) Reasonable tube sizes. One of the most commonly claimed advantages of a larger bb shell is the larger diameter downtube that goes with it. This may be an advantage on road bikes, where tubes can be incredibly thin and large for optimal stiffness. On a mountain bike, this area of the frame sees a lot of abuse from rocks and crashing, and needs to have a certain amount of wall thickness to survive actual use. Using what we consider a "safe" wall thickness and carbon layup, and a fairly typical tube diameter, we get an exceedingly stiff, light, durable product. If we used a larger downtube, we would either have a heavier frame (same wall thickness but larger diameter), or a less durable product (thinner walls and larger diameter).
5) Chain clearance. Take a look at some of our competitors frames with press in bb shells. The down tube comes so close to the chainrings that many frames have chainsuck guards on the downtube! In our mind, the chain should be able to fall off on a mountain bike and not get jammed between your crank and thin-walled carbon downtube.
6) Backwards compatibility: Many of our customers purchase a frame and build it up with their choice of parts, or parts from an old bike. By using a standard bb, we are compatible with everything without requiring confusing adaptors.
7) Chainguide compatibility: While it may seem strange to talk about putting chainguides on a short travel bike, it is becoming more common now with 10 speed drivetrains. Thread in bb's mean the frame is compatible with bb mount chainguides. We like versatility....