A thief who uses wands or staves or scrolls is pretending to be a wizard, or playing at it.
Only in the sense that the concept of the class was built around doing that. Rogues are versatile. Rogues improvise. Rogues dip into other niches. It's pretty much the definition of what a rogue is.
If that sort of "playing" is the only way the thief can keep up, something has gone wrong in my view.
But of course it isn't. As I've noted elsewhere, UMD is precisely what it's meant to be: a niche ability that becomes useful in specific situations, not the rogue's most common action. Typically, simply being able to roll trained checks in 8+ useful skills and damage the heck out of people when they're not looking is better than whatever a wizard's repertoire of memorized spells is in a given day.
That is not about niche protection (in respect of which I like [MENTION=48555]1of3[/MENTION]'s discussion). It's about preserving the feel of fictional archetypes.
An odd objection, given how the notions of a "striker" and a "martial power source" blatantly defy the feel of the original thief archetype. What archetype is worth preserving? I would think the improviser/jack of all trades is entirely worthwhile.
Yet another way in which 3E/PF departs from its predecessors, then. In classic D&D wands are awesome for wizards, because they conserve spell slots. (And many wands are MU only, eg Wands of Conjuration, Fire, Frost, Illusion, Lightning, Paralysation, Polymorphing.)
That is an odd phenomenon. This feature of 3.X magic items are an epiphenomenon of the increased importance of caster level and the item pricing formulas. Wands and scrolls are almost always at the minimum caster level, and are quite expensive (and even more so if one tries to get one at a higher CL). Their usefulness quickly becomes compromised as they are unable to penetrate any competitive SR, saves against item DCs almost automatically succeed, and range, duration, and damage, are all low.
Wands are only useful for spells for which level-based variables are not of high importance, and for which repeated castings over a short period of time are likely to be worthwhile. Cure Light Wounds meets these criteria, and a few other spells are on the fringe of that level of utility, most of them cleric spells.
I do think it would be a positive move to have wands and other spell completion/trigger magic items reflect the power of the wielder, and thus become more useful for powerful spellcasters (rather than less useful).
In general, it's better to have the wizard buy a scroll, learn a spell, rest, and memorize it when needed. In the absence of that, there is a loss of efficiency as the party must either pay an NPC, or take the middle road and get a rogue to use a magic item. The ability to select useful spells without spending a lot of money is what makes the wizard function, but on the whole, there aren't many spells that are that important.