I heard a story about some guys playing at a rehearsal and the bass cabinet blew a speaker. They placed an extra guitar speaker in there in place and they only managed to play for 10 (ten) minutes before the guitar speaker blew. So, don't do it.
The reason is in frequency range and amplitude. A bass guitar has much more power behind the low notes, even if you tune a guitar to the same tuning as bass, not to mention how guitar pickups are scaled for more midrange response, and bass pickups are more for low frequencies. If you send in enough bass frequencies through a guitar amp, there's two things that can happen, either separately or at the same time: the bass frequencies clip (too much bass range in the power amp = clipping = rip-city for the speaker), or the bass level is simply too high for the speaker that was designed for midrange (guitar).
You can picture it as a flexible material tube that is designed for a table tenns ball to go through, and you'll push a tennis ball through it. A few will go through fine, but sooner or later there's going to be a rip.. and another... then the whole tube just cracks open.