Why I dont use solder! Before you call me crazy, READ THIS!
I actually don't use any solder on my connections. I use high quality non-insulated butt connectors or where a disconnection is necessary for maintenance or upgrades I will use a bullet connector with a waterproof sealed adhesive lined heat shrink. Obviously the preference is to have zero breaks in the wire from one point to another, but sometimes this is not possible. I always install all my connections with a ratcheting crimping tool specifically designed for whichever connection I am installing.
Why no solder?
When you solder a connection, the solder moves into and between the wire strands, thus stiffening that connection. Ever tried to bend a soldered connection? Its just a hard spot. A stiff connection can create more resistance. The best path for electricity is an unbroken path from source to ground. The more breaks in that line, the more materials in that line, the higher the resistance. (You may already know this but I'll say it for the edification of those who don't know) Resistance in a wire is like kinking a hose. Sure, water is still going through it, but much less water is now coming out the other side. The increased pressure behind that kink presents in a wire as increased heat.
Bad, bad, bad, vibrations!
Stiff wire means more prone to stress cracks, so once the crack starts it will continue cracking all the way though sooner or later due to vibration and/or stress and increased resistance, and on a bike, vibration is a way of life!
Copper is very ductile, however lead, tin, and silver are much less ductile. Then you heat that copper to solder it and that is adding even more stiffness over time to the copper. Just like old wire that has had a lot of power run though it becomes brittle over time from the constant fluctuation of hot to cold.
I addition, heating the copper to solder it accelerates oxidation of that joint. Just by being in the atmosphere, it is already oxidizing at an incredibly fast rate. You ever weld something and then not long after, the area has magically become rusted to hell? The heat accelerated the oxidation. Which is outlined in the Arrhenius equation, the formula for the temperature dependence or reaction rates. Heat accelerates the reaction in most metals. Next time you get into a wiring harness with soldered connections, feel how stiff they are, some may even show signs of pretty nasty corrosion as well.
It's Fuckin Science!
Just for an example of the reaction rates affected by heat, look at how a cutting torch works. While most people think it melts the steel, a "cutting" torch actually oxidizes the metal! The flame of the torch is held on a spot until the steel is at least yellow hot or approximately 2000 degrees F, then when you depress the cutting lever, you are shooting a stream of pure O2 directly at the surface of the steel. The stream of O2 slices like a hot knife in butter going through steel plate and you can move along pretty quickly once you get the cut started depending on thickness of material. So once the reaction has begun, it requires less energy (heat) to keep going. This whole thing is an exothermic reaction.
It's a common misconception that cutting torches "melt" the steel in the cut away, it's actually oxidized away, and really quickly.
Lets wrap this up like some good wiring!
So the basis for why I don't use solder on motorcycles wiring is simple really. Adding solder doesn't improve the connection, It increases resistance, creates a stiff connection and accelerates the reaction of oxidation. It is also much more prone to breakage and failure than a GOOD crimp connection with waterproof heat shrink. By all means you should use the connection method which you feel suits your needs best, but my preferred method is based on research, experience and education.