No, you should not use it like Boxer’s dash punch. It’s got four primary uses: getting in, punishing focus attacks, OSing purposes, and carrying the opponent to the corner in a combo.
LP has too much recovery from what I can tell to be used for getting in. MP and EX let you press buttons after getting in (MP = 0, EX from 0 to +4.) Close range EX and perfectly spaced HP leave you at -1 though, so you can be punished with 1f Super/Ultra or counter hit for pressing buttons. You have to use various ways of getting in with Yun, because if you get predictable, you get punished. Lunge is no exception - it has slow start up, and can be stuffed easily if the opponent reacts in time. Keep in mind that you have to get the spacing down to take advantage of this (perfectly spaced MP Lunge may be as good as +1 while a terribly spaced MP Lunge is like -6 to -7.)
Punishing focus attacks is rather self explanatory. If you’ve got the reactions, you can pretty much stop all focus attacks. It’s armor breaking after all.
OSing generally falls under LP Lunge only because of chains.
Carrying the opponent to the corner is not used enough. cst.MP can combo into HP Lunge (including Target combo st.MP), fst.MP can combo into LP or MP Lunge. If you want a mix up attempt, you can use LP Lunge to carry and go for the mix ups. MP pushes them farther but has more recovery. HP screws up your dive kicks but pushes them even farther than MP while not having much more recovery. Wouldn’t bother with EX Lunge in a combo outside of a few characters that EX Lunge, U1 still works on.
In order of reward and effectiveness: EX Dragon Kicks, LK Dragon Kicks, HK Dragon Kicks, MK Dragon Kicks.
EX has 5f of invincibility, 4f start up. Combos into all sorts of stuff, but most importantly, it combos into your Ultra if you’re near the corner.
LK has 6f of invincibility, 5f start up. Was 7f of invincibility, but nerfed in 2012. Can combo into stuff if only the second hit connects, including your Ultra. Can go into Genei Jin with good timing regardless of hits. Won’t hit crouching opponents.
HK has 7f of invincibility, 8f start up. The hit box angle and reach make it better suited for opponents out of LK Dragon Kicks range.
MK only has 7f of lower body invincibility, 6f start up. The hit box angle and reach make it better suited for opponents jumping at specific distances to screw with your dive kick attempts (and this takes some practice, but you NEED to know when to do this if you want to make your Yun scary.)