If it isn't adhered down and you throw it in the dryer... will it adhere in places/ways you wouldn't want it to?

Nope the washing takes away the ability to adhere. I have made a few quilts with the "fusible" that I ended up useing spray basting and it washed and dried just like if it was only spray basted. Just do not try to preshrink, with out much or any agitation the fusible does not completely leave the batting.

This sounds like it is something that I don't want to use!

I love fusible batting and I use it for everything that I plan to machine quilt. I'd love to hear from someone who hand quilts, because I'd like to know if there's a problem.
I've never had a problem ironing it on my regular ironing board no matter how big the project. Just iron the amount that fits and move on, same as with a piece of fabric only you're ironing 3 layers.
Once it's quilted and goes through the wash it's just a lovely soft batting in cotton, polyester or cotton/poly blend. Actually the application process used for fusible batting uses less product than a home quilter would use using a spray and you don't have to worry about inhaling fusible batting.
I did some research on fusible batting and basting spray and based on the results I'll be sticking with fusible batting and staying well clear of most basting sprays.
Here's the thread if anyone is interested: http://www.quiltingboard.com/t-61846-1.htm

Thanks for all your help. I cut the batting to fit my wall hanging and then thread basted it so it was tight. Then I felt comfortable ironing it on my ironing board. It looks fine. I'm not real adventurous when it comes to trying something new (can you tell??) so I really appreciate all your help.