The major drawbacks of using regular speakers are as follow:
1 - Paper cone get destroyed
2 - Your amp/radio gets destroyed.
Let me explain #2
When a regular speaker gets exposed to a lot of moisture ,the paper cone will evenually soften and tear free of the outer/inner suspension. This allows the inner coil surrounding the magnet to extend further than it was designed, resulting in a possible breakage of that wire or pulling it out of the paper cone altogether.
Depending on the power/volume, that open speaker circuit causes your amp/radio to deliver more power than it was designed for, resulting in a failure.
If those same wires short out , their goes your amp/radio.
If you never get caught in a rain storm or are carefull when washing your bike then there's no problem.
Personally, I would rather spend the extra bucks for weather proof speakers as saving a few bucks somtimes will cost your more in the long run.