Well, I don't know if anyone has ever called me "reasonable" before... :joke:
But aside from the curt BRP corporate position that Bob summed up above, there are three arguments against it:
1. Function. As noted, you don't *need* a handbrake on a Spyder, anymore than you *need* a handbrake on a car. Two-wheelers obviously have good physics-based reasons for independent front and rear braking (and even then plenty of manufacturers see a market for linked braking systems). With three wheels, linked brakes do the job.
2. Form. Now, settling on one brake control BRP could have just as easily chose to put a handbrake instead of a foot pedal, but as we all know, the original marketing for the Spyder was to find people entirely new to riding, i.e. car drivers who wanted to safely dip their toes into riding. A handbrake is old hat for we two-wheel riders out here, but it was seen by BRP as needlessly intimidating to new riders-- especially as, again, there is no *functional* reason for a handbrake over a footbrake, or in addition to.
3. Regulations. This may just be me talking out my wazoo, but I imagine there are lawyers at BRP HQ who surveyed the lay of the land out there in our grand United States and Canada and determined that every little thing they could do to make the Spyder less like a motorcycle would aid its acceptance by state regulators as a unique vehicle. The thinking surely was, motorcycles have handbrakes, but cars have footbrakes, ergo, this fact among others would help convince state Departments of Transportation to register Spyders as cars rather than bikes, allowing owners to ride them with their automobile licenses.
Unfortunately, the best laid plans of 2007 gave way to the cold, hard reality of 2015, and I think that, California aside, nobody but nobody has bought into BRP's cutesy "roadster" thing, and everyone treats the Spyder as a motorcycle for licensing and registration purposes. Which means that whatever reasoning BRP once had for choosing a handbrake over a footbrake is now moot.
That said, since that decision's been made, there's no reason to change now. The Spyder is what the Spyder is.
Now, should BRP make a factory option for a handbrake? I see no reason why not, as all the marketing dreams of 2007-2008 seem to have fallen by the wayside. But they've done their market research, they know what would sell and what wouldn't, and their judgment remains, let the aftermarket take care of this if people want it. Maybe that will change someday, but I'd be surprised if it does.