In my opinion, bite/flamethrower is a good moveset for a defending arcanine. Bite gives you wider coverage when paired with flamethrower than fire fang would. It also has higher Energy Per Second generation which gives u faster access to your stab charge move flamethrower. Flamethrower while indeed having lower dps than fire blast, has a higher dodge window which makes it harder to dodge by attacking pokemon so makes for more reliable damage move. It also requires only 50 energies to use, so your arcanine will have quicker and more frequent access to a same type damage move.
Edit: I read somewhere that defending pokemon take around 1.5 seconds after animation cd to use the move again, which favors higher base power moves. If we take this into account, stab fire fang would have 5.34 dps, much higher than bite's 3 dps. I used cooldown times 2.34 and 2 seconds respectively for this calculation. If this information is true about defending pokemon then even a non effective fire fang will have 42% higher dps than a neutral bite, which makes fire fang/flamethrower a better moveset than bite/flamethrower for defenders.
