Well the azura's taught Arthur about mana preservation maybe he thought it to great a waste on his mana since he would have constantly of had to reinforce it? Also if I recall correctly only Arthur's sister Elenor could really make pure mana construxs like arrows. Also it would have made spellcasting more difficult since a mage can usually only have a few spells ongoing at a time. In I believe it was book 5 when Arthur was going to school and Lucus turns into Dranvees pupet Clara Bladeheart is watching as Lucus has like 7 in going spells and in her personal thoughts revels that the more spells a mage has going the harder it is to maintain each individual spell. It is early but I hope I was able to awnser your question correctly