Shepard is considered for Spectre talent long before the first game starts. That is part of his backstory, Akuze/Elysium/Torfan. You may not like it, but there is a reason for it.
That's no reason whatsoever why a Soldier class Shepard can drop Singularities.
Or why an Engineer class Shepard can Warp.
Or why he's, without exception, stronger than anyone else he meets in the entire trilogy, regardless of species, class or experience. Hell, he's even significantly stronger than his own clone.
The only time Tali's skills are mentioned is her mechanical and technical skills. Of which quarians are the best when it comes to hacking and computing.
Which is why I don't want to recruit her. Ooooooh wait, I have to.
Shoehorn, forcing a character onto me, the whole shebang, you know.
Liara is a college grad with no special talents whatsoever. It's been established that asari and humans are of comparable skill in biotics, and Liara has had, literally, no weapons training at all ever mentioned.
Asari are significantly stronger biotics than humans. And all of them can do it.
Besides, your squad is severely lacking in biotics when you recruit her, unless you're an adept yourself. Even a college grad with no special talents brings more biotic utility to the battlefield than Wrex and Kaidan combined. (ok, that's not completely true. A fully leveled Wrex and Kaidan bring equal biotic prowes to the squad than Liara by herself. If we ignore cooldown times, that is. If we factor in cooldown times Liara wins)