Why not glue the glass directly to the pcb heater?

Well, why not? I guess it works for me as I am using PLA with a bed temperature of 60 degrees C, not sure they make adhesives that can take the ABS bed temperatures.
Reason I'm thinking of this is I haven't removed my glass for so long now it seems pointless to have to use those annoying bulldog clips - they only get in the way of larger prints. The printer I am currently building will have a print area of 220 - 230mm square without them.
It is easy enough to clean the glass with it in place with a razor blade so why make it removeable at all?

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Richmond, New Zealand
Thingiverse ~ YouTube

The board flexes when it gets hot, that puts stress on the glass, the clips allow the glass and the board to move relative to each other.
You might get away with It, but there is a chance those forces could break the glass if it were glued to the heated bed.

Thanks, that makes sense - bugger.

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Richmond, New Zealand
Thingiverse ~ YouTube

I use diluted PVA to prevent curling and I frequntly remove glass to aid part removal from the bed. I've also proken my glass a couple of time because my Z axis end stop switch has moved without me noticing.I also wash my glass frequently in the kitchen sink to remove PVA glue.

Glueing the glas to the bed would not be a good idea.

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Using ABSPrusa Mendel Zaphod with Pronterface and slic3r 1.3.0. Printing well with 3mm PLA and ABS through 2 x J Head Mk IV b and Wade Geared Exruders. Controlled using RAMPS1.4 board running Marlin_v1.1.9

Are you using PVA glue instead of a heated bed or as well as it? I find the hot bed holds it fast then releases it cleanly when it cools.

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Richmond, New Zealand
Thingiverse ~ YouTube