I am setting up a mobile banner campaign and am tempted to use the creatives supplied by the advertiser? What are STM's thoughts on just using the creatives supplied by the advertiser?
Use them if you want to lose money 
In all honesty, I've never had success with banners supplied by the advertiser ... they are usually quite poor and even the decent ones have been abused to death by the lazy affiliates. Maybe if you were among the first few who used then, they could work ...
There is only one exception where I would use them - when custom ones are banned - but I didn't really see much success with such offers in the first place.
Definitely work on your own creatives, you need every edge that you can get in order to kill your competition ... custom creatives is one of the strongest weapons 
Hey!
I completely agree with Matuloo.
A little input as a representative of an affiliate network - don't forget, that affiliate networks ask you to work with creatives of an advertiser not because those are the most convertible, but because they are compliant with the rules of operator. So telling you to use their creatives, affiliate networks and advertisers play safe.
But this doesn't mean that you can't use your own. Just make sure you keep in mind all the restrictions for that particular offer or segment, speak with your account manager (or support) and they will approve it or help to change.