It only takes a few days of casual/light play without a PL to get a DoW/DoM job to 50.
It also only takes a day or two to get a DoH job to 50 (if you have some planning, gil, and rested/seals bonus).

There really is not a need for more XP.

There is one giant impact of having more XP however... items will be spiritbonded quicker, resulting in materia floods on the weekends. Not sure this is a good thing for the economy.

But if they removed the impact of XP on Spiritbonding, then the only 'real' benefit is that a few people who want to level a job they have no interest in will be able to do it a few hours quicker. Not sure that is sufficient reason to introduce this mechanic.

Put another way: The benefit of Double XP is so small, it may not be worth implementing.

(Also, if someone is having trouble levelling their job with the XP rate as it is, they might want to seek out suggestions/help/advice on how to better approach this world. If the problem is they do not want to actually 'play' and just want to get 50, then perhaps they can seek out PLers or other 'people' who hang out near Cedarwood and see if they can help >.>).