If the internal weak pull-up's were of no use, then MicroChip wouldn't have included them.

It depends on your application. As pull-ups for Buttons, absolutely perfect. As pull-ups for a cheap way of interfacing slow-speed devices (< 10K), even opto-couplers, perfect. As pull-ups for high speed devices (such as on an I2C bus), they're too weak. Also depends on the environment... in an industrial noisy environment, they're too weak, you need something heavier. In a domestic or non-critical commercial environment, again perfect.

Don't be afraid to use the internal weak pull-up's. I use them a lot (even in industrial applications).