If someone (generally meaning someone I don't or shouldn't trust) has my phone, I consider it compromised. Finger smudges are the easiest way to get into a pattern-locked device; this demonstrates that there are others. As JWZ says,
And if the screen locker is not secure, then it's better to not lock the screen at all: giving the impression of security when there is no actual security is far worse than having no security at all. It's a matter of expectations: if people don't expect to be able to lock their screens, they'll log out. But if they expect to be able to lock their screens and it doesn't actually work, then they're screwed.
I use pattern lock to stop my phone auto-dialling Aunt Sarah when its in my pocket, not to keep other people out. If I had a flip phone, I wouldn't have a lock screen at all.