The Taliban tried going "head to head" with the non-afghan coalition in 06 & 07 and suffered heavy casualties whenever they tried to mass. Since then they've switched tactics to focus on the ANA. They've had more success there because air support to the ANA is slower and they aren't as proficient as western forces. There's some debate as to the Taliban (Quetta Shura) strategy: Either a concerted effort to take and hold terrain (ie. Musa Qala) or simply to show the ineffectiveness and, by extension, illegitimacy of the central government and coalition. It's probably both, depending on the situation.