Here's my opinions and i don't have physics to back it up (like I usually do).
Stranded wire was developed for applications where you need flexibility, cords are the best example. Solid wire is still used in a lot of applications. Most high end capacitors use solid wire leads (there are a couple of exceptions). Most point to point wire in tube amps is solid. The advantage here is that it stays where you bend it to. I'm starting to see some interconnects made with solid vs stranded wire, woven. In fact I'm planning to make a set of my own with solid wire woven.
So it doesn't surprise me that solid wire is supplied in a speaker kit. One advantage is that you can bend it and route it away from the walls. Using stranded wire you have to be careful it doesn't fall loosely on a wall of the speaker and create a vibration.
I would do 1 of 2 things: I would either use it, or I would take thiis opportunity to buy high quality mundorf or duelund wire to rewire your speaker. I am building upgraded crossovers for my speakers this winter and I bought duelund wire to upgrade all the wiring inside the speakers while I'm at it.
Jerry