Why build yellows?

[PTTG]

I've always felt that yellow towers are a lot like green towers, but only fire once a year. I might build one or two and tell them to shoot at bosses, but that's mostly just for tradition. By the time I can afford yellow towers, I find it far more efficient to upgrade the greens and reds.

They are particularly bad at killing yellow... bows? ties? Whatever they are, the basic yellow creep. They usually arrive in swarms and while a green tower can almost always take 'em out in a fraction of a second, yellow ones take several seconds per creep...

If they charged faster as they upgraded, I might use them.
Now there's a challenge; finish a level with ONLY YELLOWS!