HQ staves arent +15, they are +15%, which when casting higher level spells is much more useful. they also give mag acc +15% (or it is believed to be +15%.. maybe +15)
HQ staves are MAB+15% and M.ACC+15%. 2 INT = 1 MAB.
BLM has +32 MAB through job traits, and a Hume BLM/RDM has something like 74 base INT, so that's a total of 69 base MAB. Add in gear:
Moldavite Earring = 5 MAB
Igqira Weskit = 6 MAB
Novio Earring = 7 MAB
Uggalepih Pendant = 8 MAB
Sorcerer's Ring = 10 MAB
Adds up to about 105 MAB.
Add whatever else you could find and whatever other INT you could find in there and you'd have well over 130 MAB, which will put you ahead of Laevateinn, not even taking into account the M.ACC bonus from Staves.
Laevateinn does have the advantage of only taking up one inventory space though. And that's probably the only reason anyone would want to get it.