I think your point is important, however it is equally important to note  the distinction between unlawful presence and unlawful entry in federal law.
The former is a civil matter, the latter is a misdemeanor criminal offense.
http://blogs.findlaw.com/blotter/2014/07/is-illegal-immigration-a-crime-improper-entry-v-unlawful-presence.html
I think the proposed (foolish) fence is an expensive feel good thingy. While there has been an uptick in unlawful entries in recent months, overall that has been declining for quite awhile. Any money to address the variety of issues at hand would be better spent elsewhere as best as I can see:
http://www.pewresearch.org/fact-tank/2016/11/03/5-facts-about-illegal-immigration-in-the-u-s/
Given that recent numbers show over half of current illegal immigrants have resided in the country for over 10 years, how does a fence solve that?
How does it solve overstayed visas and an uptick in folks here from Asia, Africa, etc ?