Glass offers visual clarity, but PC sheet has many drawbacks. To comply with building codes, safety glass is required if installed within 12" of a door, 18" of the floor or overhead. In vertical applications tempered glass is OK, but in overhead applications tempered over laminated annealed (windshield type glass) is required. In roof applications this glass cracks very easily and is very expensive. 

Since clear glass does not diffuse the light hot spots occur in greenhouse applications. Glass is also heavy, requiring a strong frame. A sophisticated install system with aluminum and rubber gaskets is required or leaks occur, especially on overhead glazing. Often numerous lites of glass are required in a roof slope, resulting in horizontal mullion bars that dam water and can easily lead to leaks. Insulated glass should not overhang the eave, requiring a horizontal mullion bar at this location and possible leaks.
 PC sheet is virtually indestructible and is inherently safety glazing! Football helmets and the inner layers of bullet proof glass are made of PC sheet. 

Multi-wall PC sheet is inexpensive and has an insulation value similar to insulated glass. PC sheet diffuses the light and is very easy to install. One sheet of PC sheet goes from the roof peak all the way to the over hanging eave, effectively shedding water.

PC sheet is a resilient material. When the Cap is snugly installed, the pressure of the legs of the Cap cause the PC sheet to slightly give, creating a watertight seal. One of the great advantages to PC sheet is that gaskets are not required to achieve a watertight seal. The more parts of a glazing system, the more likelihood for leakage. Gaskets in install system caps will often shrink, creating a gap for water to enter.