There is a 1% chance that a spider will spawn as a spider jockey so the chance for two at once is 0.01% or one in 10,000 (out of any two spiders, if there are more then the chance will be higher).
For comparison, the chance of seeing a zombie or skeleton in diamond armor is one in 15,551 mobs at maximum regional difficulty (in practice this will be much lower; your regional difficulty isn't high enough for any armored mobs to spawn. Conversely, spiders are less commonly encountered, in my experience around 5% of all mobs vs 60-70%).
Maybe they don't exploit them? I do the same since they are useless to me, other than for their original intended purpose (spawning mobs to give the player a challenge, for the same reason I massively buffed their spawn rates and added structures with multiple spawners, but limited the number of mobs they can spawn that drop loot and XP. Even Notch/Mojang has thought of doing this, see the entry for 1.3.1). They are also much more common and not as special as most players believe; I've found upwards of 20 (mostly cave spiders, but also up to 6-7 dungeons) in a single play session and have mined more than 3,700 in my first world (probably more than 4,000 since this is only since I enabled the stats for them as vanilla doesn't keep track).