Wizards can use virtually all magical devices anyway, other than divine ones which ought to be a bit foreign to them. If you want your wizard to use a CLW wand, you can still put points in the skill; you just don't get the +3 bonus.
Use Magic Device is a skill about "faking magic". Such a practitioner lacks the initiations required to create new spells and hold them on their persons, but can take an already created spell from a wand or scroll and execute the "last steps" for which no initiation is needed. Basically, UMD is the art of performing a spell's components without knowing how to make a spell.
Wizards are not schooled in this fakery, since they possess the knowledge to build and execute a spell from scratch, they don't really know how to "fake" components — they either know them or they do not. Thus, a divine spell, with components that lay completely outside the wizard's expertise, is a mystery to the wizard unless he has made an investment in learning fakery (which he can!)
I have a summoner in my campaign with high ranks in use magic device (Pathfinder game)he uses it to use items not specifically available to him as a summoner like various wands and scrolls. I also have a witch in the campaing and the summoner makes the case that the witch must us the use magic device to cast an arcane spell from a scroll. Since the witch is arcane I say she doesnt need to do a use magic device to cast the scroll so long as the spell is available in her repertoire even if not necessarily in her current spell list. If she is able to learn it at some time then its just a check of her ability vs the ability at which it was written and that is covered in the using scroll description. My take is use magic device is meant to give you the ability to do something magical like a scroll or device that you would not ordinarily be able to use due to your class or ability limitations.so then the question is do mages have to do a use magic device to cast from a scroll just because it is not in there current spell book.
From the PRD:
Use a Scroll: Normally, to cast a spell from a scroll, you must have the scroll's spell on your class spell list. Use Magic Device allows you to use a scroll as if you had a particular spell on your class spell list.
Use a Wand, Staff, or Other Spell Trigger Item: Normally, to use a wand, you must have the wand's spell on your class spell list. This use of the skill allows you to use a wand as if you had a particular spell on your class spell list.
If the spell in the scroll or wand is on the spell list for your class, you don't need UMD. So any bard can use a wand containing any spell listed as a bard spell; any witch can cast off a scroll containing any spell listed as a witch spell -- whether they have those particular spells in their personal spell list or not.
EDIT: Of course, with a scroll, there's this additional caveat:
To use a spell completion item safely, a character must be of high enough level in the right class to cast the spell already. If he can't already cast the spell, there's a chance he'll make a mistake.
Special rules for casting from scrolls are described here. But that doesn't have anything to do with Use Magic Device, afaik.
As to why it has never been on the Wizard's class skill list...
Class skills are to indicate something inherent to the training of the class. If you train at being a Wizard, you have a certain set of skills that you will have a little better understanding of.
Since Use Magic Device is about "faking magic", so to speak, it's not really something a highly intellectual person would learn from his studies of arcane arts.
A rogue knowing how to bluff or fake-out, a Bard, or even a Charismatic Sorcerer (who gains his magic through the sheer force of personality alone), all gain a little something towards UMD from their day to day training in their class.
If UMD were more about studying the item and learning how to activate it, it would likely be an Int-based skill, and be on the wizard's class list.