The answer to your question is yes. In fact, multiclassing when done properly can allow a specialist Wizard much more flexibility than they might have as just a pure Wizard (specialist or not) with some small trade-offs. One of my favorite strategies that I often cite as an excellent example of this (which many DM's have ruled legal) is a Beguiler/Conjurer/Ultimate Magus/Master Specialist combination.
The idea is to ban enchantment and illusion. Although illusion has a lot of good defensive spells, the Beguiler spell list contains virtually all of the best illusion and enchantment spells which makes banning those two schools relatively painless. The Ultimate Magus part of the combo allows you to boost SKL and ECL for both of your arcane classes, as well as use persistent spell more often without raising the spell's level. Additionally, Wizard and Beguiler mesh extremely well together because they share the same primary ability (intelligence). Finally, the Beguiler's stupid amount of skill points (though still not as high as a Rogue's) and the inherent Trapfinding ability will really help you be more self-sufficient. If you want to follow this path, I recommend taking your first level as Beguiler, the next four levels as Conjurer, Levels 6-15 as Ultimate Magus, and the rest of your levels as either Conjurer or Master Specialist. If you execute it correctly, and take the Practiced Spellcaster feat for both Beguiler and Wizard at the appropriate times, you will end up with a character who has the spells known of a level 17 pure Wizard (which allows you to cast up to one level 9 spell per day) with a caster level of 24 and the spells known of a level 10 Beguiler (which gives you access to level 5 Beguiler spells) with a caster level of 18.
Some might say that the effective loss of a few Wizard levels is simply too great a sacrifice, but I believe that as long as you are capable of casting 9th level Wizard spells the addition of essentially free persistent spell usage as well as a bunch of super useful utility spells that don't need to be memorized to use far outweighs being able to cast three more 9th level spells per day. As always, use your own judgment. Another variation of this strategy involves using the Sorcerer class as the base and combining it with Knight of the Weave. To qualify for Ultimate Magus you would need to take the feat that allows you to prepare your spontaneous spells ahead of time but fewer DM's permit this as you end up with a Sorcerer who has full spell progression, a small number of useful divine spells, and no down sides. In order for most DM's to allow your build in the campaign, you often have to show that you are giving something up in exchange for something else. In this case, you are trading a few spells per day from your highest level spells for the ability to specialize in a particular school of magic for its benefits while still being able to cast some of the best spells that belong to your prohibited schools.