Good point ... but it remains to be seen if the items available to be purchased with sol/poe will change, so if you don't need the tomes for gear just yet, there is no harm in capping. Plus it will make capping later for when we can exchange easier.

Materia is pretty much a good investment, but I don't know what the effect of tier V materia will have on prices. You could make a loss if you blow a lot of gil on IVs only for their rarity to decrease/value to weaken.