Using boosters defeats the purpose of spaceplanes. The idea is to get to Kerbin's high atmosphere with a high speed using only the extremely efficient jet engines, and with lift surfaces to fight "gravity drag". Once there, when there isn't enough air for the jet engines to work properly (or at all), you turn on rocket engines to reach orbit proper, for a fraction of the delta-v that it would normally take, and without having to drop stages.
The main advantage is that it's dirty cheap, once you get a working spaceplane: you're only paying the price of fuel. It's also fun and/or pretty to fly.
It's super hard to make it work though: you need to understand lift, the optimal working conditions of jet engines, an ascent path that allows you to gain speed before going for orbit, the proportion of delta-v to be produced from jet engines and from rocket engines, etc. etc.
You can also do SSTO (Single Stage To Orbit) rockets. Kind of what Space X is trying to do. They are considerably easier to make them work... which makes them less fun, duh.
Honestly though, you need the better jet engines to make spaceplanes useful, so either the whiplash or the Rapier. Other jet engines die at "low" altitudes and speeds, which means rocket engines still have to do most of the work. They are a lategame thing.