[guineapig]

In other words, yes they do server a purpose.

Besides stun whips and the like, mages get a summoned sword that does dirrect damage versus summoned mobs (ideal for certain parts of SolB).

The summoned weapons that have resist bonuses can be useful in certain situations to extend the life of your pet. (+MR, PR, DR from the dagger, +FR, CR from the spear)

Staff of writhing adds a sick amount of agro to your pet as well as some great additional DPS + a stun component. It's such a great soloing tool and pretty easy to get as they drop probably 10 times a day (at least) at Royals.

The root swords (previously mentioned) are a great way to give the crowd control capabilities of the earth pet to the more DPS producing pets like fire and air. If you are in a group with no bard or enchanter one of these swords in your pets hand can come in extremely handy not only for minimal crowd control but also for agro control. Plus in group situations mage pets tend to last for ever.

Will you be using these special weapons every single time you summon a pet? Doubtful. But at the very least I would give my pet 2 summoned weapons for the additional resists, a summoned coldstone for the +5AC and 2 summoned levitation rings for another +8 AC. I have not parsed the difference in damage taken but I'm willing to bet that there is a measurable difference.