Very good observation.
1) Touch DNA on the phone, too...
2) It characterizes BG as the guy who reads a lot on the topic of true crime.
3) Would BG even risk touching Libby's cell phone, though? For the same fear, DNA...
4) If BG traveled to Delphi in car/bike, he probably took his phone with him and left it in the car. I can see some benefit in leaving the phone with GPS turned on at home (it establishes your alibi). However, these day and age traveling without a cell phone is terribly inconvenient. LE said BG was moving around very fast, meaning, he needed his cell phone more than we think. So his cell phone should ping somewhere around the trails area on that day from 12 to 5. MOO.
I can imagine BG buying a burner phone for that activity, but if he is a family man or has parents, or is a kid, or needs establish a good alibi, answering his own cell phone would be important. (Or can he connect a burner phone to own cell phone? I am sure he can, but it can be traced, too).
I still think there is a possibility he left his own cell phone in the vehicle. Can it be researched somehow? Can all cellphones responding in the area around Monon bridge that fateful day from 12-5 be traced?
5) Either way, I see BG as the person who reads and Googles on true crime, is the member of the forums, has some knowledge of GPS and cell phones; if Libby went to crimininology classes, perhaps BG attended similar classes at another or the same place as well? He has to have the trail of exploring the issue of crimes, have the knowledge of similar crimes, and I think he never concealed his interest.