A few of us were curious as to why Glowforge does it that way since most other lasers use a fixed mount? 
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GF Team must have a very good reason, but I don’t know which one 
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My friend, Patrick Hood Daniel, did the same thing for his laser cutter kit – he explained it was to reduce the complexity of using mirrors, if memory serves. buildyourcnc.com
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Less (distance) is more in this case.
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I think @Dan has discussed this before. It’s to eliminate fouling issues between the exit from the laser & first mirror, almost totally eliminates alignment issues between mirrors and because it looks cool so you can watch the purple glow while lasing 
I look at it as prevention of user problems just like HP did with laser printers. No real significant difference between a laser printer and a photocopier. But photocopy machine maintenance is all horror stories. HP figured out that most problems were due to convoluted paper paths and issues with imaging drums. So they made all the problematic parts into a single disposable consumable part (the print cartridge) and we’re able to streamline paper paths. Almost no one needs laser printer maintenance repair people.
Same thing with laser cutters - two biggest operational issues are beam alignment and dirty mirrors & lenses. Cleaning often messes up alignment. So by putting the tube on the gantry they’ve eliminated a couple of mirrors needing cleaning and aligning. According to Dan alignment is no longer a thing to worry about. Cleaning they took care of with their window trick and the removable he’d makes any lens maintenance easy and doesn’t mess up the alignment.
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You forgot the most important part! It looks cool!
But in all seriousness, good writeup
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What about jostling or vibration of the tube itself? Wouldn’t moving the tube like this shorten its’ lifespan? Or does it affect it at all ( or is minimal?) That’s what I always wondered, since the glass tube is the most fragile piece subject to breaking on the machine - seems it would behoove you to keep it stationary and protected. I’m more than positive that the GF team had gone over pros and cons on this issue before deciding on the design however ( Just was alwas curious). 
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The tube isn’t particularity fragile because it doesn’t have a filament like a bulb or a thermionic vacuum tube. It is just glass with some electrodes and mirrors plus a glass spiral for cooling. So unless it strikes something, it should be fine. The acceleration of the heavy Y axis is not going to be high enough to stress it.
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@jamesdhatch - spot on.
Not measurably - @palmercr is right. As long as we don’t exceed certain parameters of force (which requires a bunch of specialized computation, but we do that too) the tube is fine.
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Does that mean that the travel speed is slower than what it would be if the GF’s tube was fixed (unmoving)? I’m also wondering if the added weight of the laser tube on the moving gantry affects inertia to the extent additional space is needed at the end of a pass for the gantry to slow down and reverse direction and if dealing with that inertia is responsible for the somewhat reduced work envelope compared to the original design specs.
To be clear, I’m not necessarily trying to play Monday Morning quarterback so much as understand the compromises that have gone into the GF design.
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Trying to design your own laser?
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It would have meant that, if we hadn’t doubled up the Y-axis motors to compensate.
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No, just exercising my mind until shipping is announced.
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