Guilt or Innocence?

You need a defense anyway.  Just because you really committed some or all the facts alleged in complaint doesn't necessarily mean that you are responsible for all or even some of the damages sought in the complaint. 

You may still have legal defenses.  For example, the suit might have been filed after the statute of limitations expired.  Also,  Congress or your state's legislature might have substantially limited the types of damages that can be collected in your case. There might have been a recent Supreme Court decision that dramatically changed the law.  You probably won't know about your legal defenses unless you see a lawyer.

Finally, by vigorously defending your suit and making the plaintiff prove each and every allegation you might encourage settlement on more favorable terms.  Just remember, a defendant wins merely by not losing. This is a huge advantage.  Use it.

There are many more reasons to defend a case; there are very few reasons not to.