Cons:
1) not obvios;
2) time-consuming;
3) involves additional implementation difficulties;
4) little "out of concept" (we have no other objects that act the same and nothing like this was discussed to this point, as far as I know);
5) when you get the staff you already have one hand for it - how do I know that it is not useable alredy?
6) how do I deselect staff when I always have one hand for it?

Sorry, I can not find a single pro for it except for the "solution" of secondary use problem

Sorry if my answers seem a bit harsh. I do not intend to criticise everything posted here - just to find the best of all possible solutions. But I think this is my responsibility (as well as responsibility of other community members) to denote things that are not very well according to my opinion. And I suppose I make my point pretty clear.

In fact there are a lot of solutions for this problem. It is just that I 100% like neither of them:
1) We can have seperate staff for every spell present (limiting the number of spells should be in order). The staff is intended to cast the spell immidiately after it was used. The spell which is cast by a staff is defined in the "Magical Workshop" before the staff construction. Though it solves the "secondary use" problem it leads to extreme complications (50 different staffs lying around from which you should chose the one you need at the moment)
2) The spell can be selected every time you use the staff (limiting the number of spells should be in order as well). When you use the staff, a (ring) menu appears from which you choose your spell and either spell is cast immidiately or after the next use. It also solves the problem but involves huge spellcasting difficulties (not to mention time consumption) which will probably make most of players (myself as well) abandon spellcasting for good.
3) We can make an actual spell something like an ammunition for the staff. Spells can be contained in crystals which are built into the staff (or scrolls put into the book). When you place a crystal(s) into the staff amunition slot the staff can be used. When the ammount of crystals runs out you should insert another crystals there. Thus it is rather simple to switch spells for they are changed the same way the musket/bow ammunition is changed. And this leaves an empty secondary slot after ammunition was placed. Also it saves us from nessecerity of implementing "mana" and this kind of stuff (which I would greatly like to live without). Also this idea provides the most obvios solution for using spells (because the same sequence of actions is used for musket, bow and other weapons).
4) We can also implement a third special slot which is shown in different color and only available if the staff is carried. You can use it to select the currently active spell and then cast it by using the staff slot. When staff is dropped, the third slot disappears. This solution is ocular, the player will easily associate the sudden appearence of the third slot after picking up the staff with the possibility of choosing the spell. We can use both mouse buttons for spellcasting and have an additional slot for our shovel/d-box/the-most-beloved-item.

I only wanted to show that the first solution that comes in mind is not always the best. Again, sorry if I offended anyone.