How do you suppose people, having chosen a lawyer, stick with someone for years? Trust, believability.
Call it what you want. In short, credibility.
Credibility is so incredibly important and fickle, mostly because it's so easy to lose and so impossible to regain.
When you're dealing with clients, if you ever ask yourself if it's worth it to do anything but give your all to them, it ain't. Even if you don't lose these clients, you'll lose an endless stream of referrals that would have issued forth from them.