If I didn't have any target events for a year or more out, I wouldn't do any short-duration aerobic efforts until I got within about 3 months of my first target event. I would do long L4s almost exclusively if my training routes were conducive to such efforts. However, some riders just don't enjoy the boredom of long L4 efforts and prefer shorter, harder efforts. While the aerobic capacity benefits are probably less than with concentrated L4 efforts, one can still make substantial improvements in aerobic capacity with lots of anaerobic efforts (e.g., L5 and L6). So, to net it out, the biggest bang for the buck is lots of long duration L4s, but if that's not to your liking, you can do a lot of good with lots of anaerobic efforts.