Staffs counting as two set pieces was changed for bows and two handed swords and other melee weapons too.They actually weren't always considered 2 handed weapons. Not until Summerset. Previously, they only counted as 1, although you still couldn't equip something else with them. Same for bows and actual 2-handed weapons. That patch (update 18) they changed bows, staves, and actual 2-handed weapons to count as 2-pieces so people using them would benefit from a full 5 piece set bonus, instead of 4.
Allowing staves to be equipped with something else would require a new weapon line with how the game works. Right now we have 2-handed, Dual Wield and Sword & Shield. If you only equip a single handed sword/dagger/etc, you won't benefit from either skill line (need to equip two 1-handed weapons to use something like Flurry or sword and shield to use Puncture, for example). So for technical design, staves needed to count as 2-handed.
there is a "Bo Stick combat", if you watched Xena, the warrior princess, her sidekick, Gabrelle use as Bo Stick., when she walk, she lean on staff when tired or resting, when she fights, she use 2 hands.
I am kinda curious why the staff is a 2h weapon.
You hold it in one hand, it has single hand animations, you can preview a staff and shield equipped, and it has base weapon damage that follows 1h progression and not 2h progression.
Also, i know that this isn't skyrim, but staves were 1h weapons in Skyrim too so there is reasonable precedent within the series (and yes I know that 2h staves also appear in the series).
I feel like this is a missed opportunity. Instead of making Frost Staves the Magicka tank weapon Zenimax could have done this:
1) Made 1h weapons with empty offhand have their own skill line (preferably a melee magicka/stamina "spellblade" style hybrid that scaled on the higher of your two resources... you know... the MOST advertised single weapon style in ESO marketing that for some reason isn't in the game)
2) Left Destro staff with empty offhand the same but reworked Frost into a DPS option
3) Made the tanking skill line dependent upon equipping a shield with either a staff or 1h melee weapon. Skills and passives could split slightly (similar to how Destro splits on staff types) but have the same utility (for example Puncture would do the same thing but would use either Stamina or Magicka based on equipped weapon and would do either Physical or Fire/Lightning/Frost damage)
This would give Magicka Tanks a fully fleshed out tanking tree to use while also allowing DPS frost mages to be a thing.
Most likely, a person using a staff/shield combination would end up using the staff like a blunted spear. They would probably hold up the shield and stab out from behind with their staff. It would be a weapon of desperation rather than practicality, imo.While you could hold a 6-foot stick and a shield at the same time - you could not use either one effectively without dropping the stick... So even if the stick is just an overgrown wand - you're going to drop it the moment someone bashes you because of your leverage being all wrong.