Feedback, bleed, clarity in the mix. All these apply to both miking an amp and miking the guitar directly without using an amp. These apply to using microphones in general, in any instrument, with or without amps involved.
It depends on the venue (dimensions, materials, shape), the equipment (mics, amps, PA, monitors), and the performance (# of instruments, type of instruments, the mix). In many scenarios, there is a very big potential of feedback. It doesn't need to be a complex scenario. One guitar alone can have problems in a small and reflective venue (both standing waves and reflections being an issue).
Mixing engineers know this, and they often recommend to use direct out in all or some instruments. It also makes mixing simpler, since you don't have to worry about bleeding (which again, depending on the scenario, can be a big issue).
When he said that "it can't lift up the signal", I'm sure he meant that he can't bring the guitar to the amplitude levels he wants because it would induce feedback. This is why feedback can impact your performance even if it's not manifested as feedback, it will be manifested as a limitation of volume (which will vary among mics and instruments), this will be reflected in the mix. Some instruments won't be able to sound loud enough to clearly be heard (this is because feedback will be triggered at the same volume as the instrument needs to be heard through the mix).
What might have happened there is that the soundman noticed that he wouldn't be able to capture the guitar correctly with a mic (volume, tone, dynamic, whatever), and he considered that by using line out he could achieve a better individual sound and/or mix as a whole. It's a pros and cons game. You lose the guitar amp, but you gain in other areas. How good is miking your amp if the audience won't be able to hear it? At that point it really doesn't matter how beautiful it sounds, all the details will be lost since it won't be able to sound loud enough, or the details are being masked by other things going on around the mic (as good isolation is not always possible).
If you find yourself in this situation a lot, and you don't like the sound of raw line out or whatever process the soundman chooses, do yourself a favor and have amp or speaker-mic modeling as an option, as a plan B, in your performance. There are a lot of options out there, in both hardware and software form. This way if amp miking can't be done, you will still be able to have the amp sound.
The most popular sepeaker-mic simulator (that will simulate the speakers and mic combo, but not the amp), is Hughes & Kettner's Redbox. You feed it with your amp's output, and it simulates being the speakers and mic. I see this one the most often since it's inexpensive, easy to use, and it's hardware (doesn't require you to carry a laptop around).
For amp-speaker-mic simulator there are a lot of options. The most populars are Native Instrument's Guitar Rig, Waves' GTR, and IK Multimedia's Amplitube.