I'm not 100% if that still applies (and also unsure if it applies if you're melding materia that doesnt "fit" your role, aka battle-materia on a crafter), but afaik gear thats been melded has an increased spiritbond rate - people used to meld cheap materia into gear all the time for faster bonding.
So... maybe he was trying to do that? If DH+2 was the cheapest materia he could get his hands on (or already had his hands on), he could increase his crafter-materia-output at minimal cost.

...thats at least the only explantion I can think off.