I called out some Liara fans for denying relatively obvious flaws/inconsistencies and that point seems to be reinforced when some of her fans simply argue for the sake of arguing or voice opinions as undeniable facts (i.e. her combat usefullness). Or of course when some of the more arrogant ones start proclaims of "canon romance" or "flawless/perfect character".
However, she is used to defending herself on remote digs—sites are often raided by privateers for artifacts—and in combat, she makes a formidable ally due to her near-complete focus on biotics.
One lonely Asari vs privateers. I think that qualifies as having combat experience and usefullness.
I'll concede her transition to information broker is improbable, though she was raised by her mother, the powerful and important Asari Matriach, and everybody in Asari society expected her to become just as important as Benezia (which was the reason she became an archaeologist). That will at least have given her some experience with the nastyness of high level politics.
And no matter the improbability, it doesn't bother me in the slightest, just like I'm not bothered with Legion's/the Geth's 360 on opinion of life, Wrex's transition from careful, intelligent Krogan to careless, brainless brute, Mordin's 720 flip, Jack's full-blown-crazy to caring teacher or all that kinda stuff.
**** happens, people change.