A sight glass on the liquid line before the TXV would let the technician know if any liquid flashing is occurring before the valve. This flashing could be from loss of subcooling or too much static and/or friction pressure drop in the liquid line before it reaches the TXV. There is a big difference between a bubbling sight glass and a low flow rate sight glass. If bubbles are entrained in the liquid, this is sign of a pressure drop causing liquid flashing, or an undercharge of refrigerant causing vapor and liquid to exit the receiver because of no subcooling
There's never any sub-cooling in a receiver. You've got gas on top and liquid on the bottom. By definition you have a saturated condition in a receiver.
MintJulep
Never say never but for the most part you are correct if the receiver is in a hot ambient, sub-cooling may be lost as refrigerant sits in the receiver. This is why some commercial systems have receiver bypasses for certain situations. Receiver bypasses are nothing more than a liquid line solenoid valve controlled by a thermostat, which will bypass liquid around the receiver to the liquid line.
A little off topic but...
When a system is operating, subcooled liquid enters and exits the receiver quicker than the pressure and temperature can achieve an equilibrium. The liquid/vapour interface is the only location where saturated conditions exist. This is why when you measure the outlet temperature of a receiver it is almost always subcooled; in reality, most of the receiver is full of subcooled liquid if the unit is operating properly. Some systems do bypass liquid to gain some efficiency, particularly if they have mechanical subcooling.
As imok2 said, a sightglass is used to determine whether a solid column of liquid is present in the liquid line. This helps in troubleshooting many problems such as refrigerant charge, lack of subcooling, plugged filter/driers... etc
