Not only will you be able to hold up the attraction for all the other paying guests, but you will inevitably annoy the cast member who will "gladly" take the photo for you!
All joking aside, I really do love taking photos in ride vehicles. The only time we actually ask a cast member to do this is if its first thing in the morning and there is no existing line of people waiting to board. Spinners are great examples of how awesome these photos can be! I especially love the brights colors of Triceratop Spin!
These photos are great for when you're touring the parks solo! Its probably just my imagination, but I'm pretty sure the cast members take pity on people touring solo, and are exceedingly chipper to take your photo.
I utilized this method quite a bit while Adam was at a conference at The Contemporary and I had to galavant around WDW by myself. Poor me.
Ride vehicle photos, taken by someone else, is a fantastic way to commemorate a favorite ride! And is a terrific alternative to on ride selfies, which never come out very good, and are generally pretty dorky. But always remember not to execute these at busy times of the day, because you honestly don't want to ruin anyone else's good time.
ALSO, I know that everyone who reads this blog is pretty smart...and I shouldn't have to mention this...but DONT EVER ask a cast member to take your photo on an omnimover. It goes without saying that this is all but impossible and fairly dangerous.