I agree that border security is important. I also agree that the border should be secured. But how will we know when it is? On January 28, 2013, McCain simply told reporters, "I'll know it when I see it." Come on.
Second, as a practicing immigration lawyer - what seems to be missing from the secure-first crowd's argument is that securing the border - however you define it - is related to immigration reform. Inextricably related. You can't do one without the other. In fact, you will secure the border by passing meaningful reform. Here's how.
If reform is passed and people have a means to immigrate legally, most folks who would have paid a coyote at least $3000 per person (usually closer to $5000) to cross the border, risking getting shot by cartels and gang members, or dying of thirst or hunger, will certainly have much less an incentive to EWI (enter without inspection.) That means less people crossing the border. To the extent a "secure border" means stopping people from EWI-ing - that means it will be easier to make the border more secure.
What about folks who want to flout the law and enter illegally anyway? The fact that the border is more secure means that the would-be lawbreakers will know that there is a much higher chance they will be caught. That further chills illegal immigration, which makes the border that much more secure.
Trying to secure it first while we have a broken system is like trying to run up the down escalator. You're going to expend countless resources chasing an illusory goal, working against the flow and moreover, you'll never really know when you're there.
If the secure-the-border crowd was really serious about securing the border, they would come out enthusiastically for immigration reform.
There must be something other reason they're not.