I always ran transom straps to the trailer frame on my older stuff with full length trailers, but the current trend in small trailers is to have the bunkers extend much longer than the actual frame. Going from the frame over the top of the gunwhales feels more secure than attaching to a piece of possibly questionably mounted 2x4 bunker. Then again, I've seen several tinny's with crushed gunwhales were somebody got carried away strapping it down.
(edit) Side rails would be nice but cost money and weight... my 14' is just over 100 pounds so the big benefit is not guiding it on the trailer so much as keeping it off the guy next to me's trailer if there's a breeze. Moving the boat side to side on the trailer is not a problem, keeping it from moving is. lol.