1. The Power Of Improvisation.
A weapon imbued with the power of an unwilling spirit can do very few things. This analogy is like (if you've seen Clone Wars) a droid to a Jedi. Both can follow orders, the clone being better at the taking orders and far easier to produce than a Jedi, taking hours to produce compared to the years of training of a Jedi. Yet Jedi are still vastly superior to droids - why?
This is because of one and only one reason. Jedi can think for themselves. Sure a tactical droid can create maneuvers, but the Jedi can think on their feet, something which a droid simply can't do.
The droid is comparable to the imbued object, whereas the Jedi is comparable to the contractually summoned spirit. Your object has a limited capacity, it can only do what you tell it to do. "Go do this", you say. Well, it does that, something unprecedented happens, your object is done for. But the spirit, well it can improvise, and fulfill the objective a different way.
2. Intimidation and Status.
A true mage is not one to use weapons like swords or maces; a true mage needs only his mind to crush his enemies. The wannabe mages running around, well they don't have the skill and experience of a true mage, and they don't have a true mage's attitude. Also a mage - a good one, at least - should be benevolent and diplomatic, a real mage does not need to trap a spirit by force to obtain its power, a mage will use his greatest weapon - his mind, to make the spirit join him of its own free will.
Also, skilled mages who are capable of completing the ritual, understand that the easiest way to win a battle is to intimidate your opponent out of fighting. So imagine this battle on a vast plain. One one side, we have Wannabe Mage and his spiritual weapons/tools. On the other side, we have a real mage who... appears to be alone. Wannabe mage chants a few words and his weapons fly to his side, pointing at the real mage. The real mage slams his staff into the ground, an army of spirits appearing beside him. Clearly, at this point, the real mage has already won, but let's give wannabe mage the edge.
Wannabe's magic weapons defeated the spirits. Now real mage is dead... right? Wrong. Real mage starts chanting and twirling his staff, rising above the ground, emanating blinding blue light. The light is gone, the mage simply looks at the wannabe sighing, he magics up a chair and a glass of mead, and watches. The wannabe's weapons clatter to the ground, magic, energies, and auras swirling around, coming out of the weapons. The energies condense into spirits, angry ones, emanating anger, and power. Eyes glowing red, snarling. At this point, the wannabe runs. The real mage being benevolent calls the spirits off.
And the third and last reason, as evidenced in the previous paragraph, is the inanimate objects act as a cage for very unwilling, angry spirits. Any experienced mage will simply grab the attention of the spirits with a simple true-light-emanation spell, makes them contractually serve him, then all he has to do is overpower the containment magic keeping the spirits trapped, and the spirit trapper is done for, then he simply frees the spirits and the battle is won, with little effort from his side.
Hope this helps!