from my perspective:
pros:
you can enjoy riding together, without riding 2up (2up sucks imo)
having the same hobby means talking about it, looking at bikes/gear together
girls look good in gear
girls that smell like motorcycle rock
I have better judgement/make safer decisions when we ride together
we can enjoy washing the bikes/maintenance together
she loves it when she sees our bikes parked side by side
cons:
she might ride a faster bike than you LOL (in my case gsxr750)
she might not like the same type of rides: ie. she loves commuting on her bike and riding in perfect weather. If I want to get up early on a hot day and ride before its too hot, she's likely to opt for sleeping in then sitting around sipping coffee until its too hot out for her to ride geared up. Or when I'm feeling like I'm just getting into the ride she is starting to get tired of sitting on her gixxer.
she will be hit on tenfold when on her bike or in gear (doesn't bother me, but might some)
I worry about her when she rides alone
On a side note. Women who ride are just as bat poop crazy as those that don't. Since they are all borderline psychotic, you may as well date one that rides.