Whether a defrag will do much for your particular use or not.
With 70% free space - perhaps, perhaps not.
Over 3 years of use, the free space could be fragmented pretty good and I've seen boot time and app launch time improvements with iDefrag on 75% free space drives - but, I do move a lot of data on and off my system drives.
In my own testing, iDefrag even improves upon a clone restore - both in boot time and app launch times. Download the iDefrag demo and check the Layout tab at the bottom when it gets done - that provides a picture of where on the drive the data is stored and where the free space is located.
The pic below is of my SSD - if that was a pic of the HD in my other Mac - I'd be running iDefrag on it tonight for sure.
Actually, I can be pretty anal about slow downs and probably would have defragged it long before it had data spread out from one end of the drive to the other like that pic shows.
iDefrag does take longer than a clone restore - so it's something that I typically only ran at night.