Why should I use a highway ramp?

If you use any multi-lane one-way road as an exit to a highway, you effectively reduces the highway to two lanes at that point. This is because the lane on that side of the highway typically becomes turn-only, while the middle lane will become turn/straight. Unsurprisingly, this can be bad for traffic flow on the highway. Note that this also applies for splitting a highway into two highways.

You will find similar problems when merging a multi-lane one-way road into a highway (again, this includes highways merging into highways), with traffic trying to merge across multiple lanes and potentially interrupting the flow of the highway.

Lane switching (to avoid turn-only lanes or merging traffic) has a negative effect on traffic flow on any highway with a moderate throughput of traffic. This is why junctions like this exist.

Other considerations:

• Faster than all one-way roads, with a limit of 80 (down from 100 for highways).
  Note that this is twice the speed of two-lane roads, and thus should have about the same overall traffic capacity (at least while traffic is moving at full speed)
• Cheap to build (30/cell; cheapest of all roads except gravel)
• Cheap to maintain (0.32/cell; same as regular two-lane)
• Cannot be zoned
• No pavement for pedestrians to walk along
• Traffic will never park on them

With all that said, personally I tend to use ramps as the entry/exit road from highways everywhere, but will often have exit ramps change into one-way two lane roads before they reach a junction, this can help the traffic to spread out as it waits at the junction and hopefully not back up down the ramp as far.