Well, as I see it, the crux of the issue is that Merak doesn't like the bookkeeping to track who is carrying around what. And really, how many of us do like that?
By the book, Detact Magic will show the strength of the aura. That's all. They can guess at the range of the enchantment, but they still don't have specifics. Identify is a simple solution to offload the bookkeeping back to the players.
Because Detect Magic has been used differently in this campaign, it is now a psuedo-replacement for Identify. You still need identify to obtain command words and figure out what many wonderous items do, but you don't need it for the specifics of a magic weapon.
My advice: If it works and everyone is happy, don't sweat it. Technically, it is not by the book, but that is the biggest drawback.
Now, if you want to change things, consider running it strictly by the book. Detecting magic on a weapon will give you one of 4 aura strengths. They can guess from there. If they want the _real_ answer, then they need to use Identify.
As an alternative, make it a spellcraft check to differentiate between a moderate +2 and a moderate +3 aura.
For added fun, specificy that the weapon is only as good as they have determined it to be. It is +3, but they biff the spellcraft check, it functions as a +2. Though, this solution is kind of silly and is really just a way to encourage them to use Identify.