Lugra +1 vs Ishvara

Greetings! I often see when reviewing WS sets for jobs that can use Lugra +1 that it is rarely used and I'm really struggling to understand why.

A lot of the WS for those jobs scale either 80+% with at least one of the stats which will be 8-16 depending on day/night or 40% with 2 of them giving you 16-32 stat bonus not to mention STR for overcoming enemy VIT but many of them opt to put Ishvara in the companion slot with Moonshade.

Is WSD really THAT valuable?

I haven't seen much information on the ratio of WSD/FtP vs Scaling Stat contributions to overall damage.