Slower than a hop kick and way less damage potential compared to Lars/Bryan u/f+4, Dragunov's u/f+4 is extremely lackluster. As such it can be in your best interest to sacrifice a little bit of damage for oki options, bringing the damage more inline with the rest of the cast. d+3,4 spike oki gives you that chance.
Credited to tyler2k on TZ: http://www.tekkenzaibatsu.com/forums/showthread.php?action=showpost&postid=4145131
From my "Dragunov Practical" video: http://www.youtube.com/watch?v=cM58pbzGcgw#t=0m12s
In DR, Dragunov didn't require forced KND oki to get most of his damage as u/f+4, SSR, d+4,4, d+4,1,3, provided both carry and decent damage. While there have been many improvements on Dragunov from DR to BR, his staple was removed (though never replaced) leaving u/f+4 one of the worst jumping launchers in the game. While d+3,4 is a band-aid fix to making u/f+4 decent, something more permanent is needed for T7 and beyond.
Update: Lambech from TZ brought a good point up in a comment on my "Dragunov Practical" video, while linking through this site, the combo/option is referred to as u/f+4, d+3,4 but in actuality the combo used was CH d/f+1,4, d+3,4. Both options are valid and provide identical oki options post d+3,4.