I don't think it has anything to do with rectilinearity: bricks are rectilinear, and I don't see anyone complaining about yet another building stuck with rectilinear bricks… or rectilinearity in any other material for that matter. The vast majority of what is being built is still dominated by the 90° angle. Curves and freeform are still the exception.
Colour, texture, and perceived cheapness are more believable causes for the debate on spandrels.
Colour: I do not agree that most designers are going bonkers with spandrels. My experience is that most spandrels being installed are one shade or another of grey or beige, and that colour is still being used timidly for the most part on most projects. There are buildings where spandrels are being painted with more playful patterns now, like here at Motion, and on the east facade of Queen-Portland, and more vividly now, like at the new YWCA, and at the Paintbox, but these are still the exceptions. Whether you like the patterning or the vividness is subjective. Which colour applications appeal visually to you? How colourful do you want buildings to be? These are questions of aesthetic taste and of the value of the dignity or the playfulness or the contextual appropriateness of any design, and they can be argued ad nauseum without all parties arriving at agreement, ever.
Texture: It would be difficult to find anyone, even the most ardent supporter of minimalist modernism, who would argue that a city would benefit from a monotonous application of flat, reflective panels, whether of clear, see-through vision glass, or of translucent glass, or of reflective glass, or of opaque spandrel, on building after building. We want, on the whole, variety. We want the delight of contrast. Whether the contrast needs to occur within or between buildings however is the debate here. Certainly bricks and windows set each other off nicely… but Mies van der Rohe and others taught us that simplicity and the harmony of the whole can be an equal pleasure. When architects (or developers) do not want to offer all-window exteriors, spandrels can be used to reduce the complexity of an exterior if minimalism is what the designer desires. Contrast and variety can still be achieved in a city with minimalist buildings as long as they are not the only buildings going up.
Expense: Spandrels are usually cheaper than most other opaque cladding alternatives, granted, but in fact there is a range of build qualities and prices. An example of a more expensive spandrel system is what's going up at the SickKids Research tower now: the double layer system there has the base colour applied to the inner layer, while the outer layer has a translucent frit baked on it, allowing patterned views directly through to the inner layer. That gives depth, looks great, and no-one is complaining. It's tougher to tell from a quick look how high the build quality is in most cases though, but the complaints come when spandrels look cheap, and then all spandrels get tarred and feathered by the same brush. We should and do hold builders accountable when the cheapest materials are employed, and not doubt we will continue to push for higher quality work and materials when we see problems. Higher quality can be achieved with better spandrels.
Spandrels are not going away any time soon. The City wants condo towers to be more energy efficient. They do not want so many all-window buildings. LEED certification if more easily achieved with a 60-40 opacity-transparency split, so more exterior walls are coming. Some developers will continue to demand spandrels, and some architects will continue to volunteer them. UrbanToronto members will stay vigilant; I just think it's more worthwhile to demand good spandrel than to fight spandrel all-together.
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EDIT: not all of that is aimed at your post directly Parkdalian, that's just my take on the whole spandrel debate.