Acid is useful in many situations, but like I said, if you don't adapt to the situation it doesn't matter what you have.
For example, last night I had several Tigers (Acid/EMP/Thor) defending my base, but my opponent was trying to get past my defenses by mass-producing EMP Lynx with a few RPG Lynx thrown into the mix (it was about an 8:1 ratio). In order to win I had to mass-produce my own EMP army and then counter his swarms by pumping out Acid (and Rail Gun after I ran out of rare. Nostalgia's rare sucks).
Just goes to show that you never know what you'll need to use to win.