Balanced builds are the shit imho.
Str builds are situational in that they are great for ganging and being "line infantry" - when things are going as they should.
Problem is that when shit hits the fan and you need to gtfo/fight multiple enemies/duel/dodge a lance then str builds are utter shit.
Also, they're incredibly boring to play imo.
Agi builds are situational in that while they are great for picking your fights and getting out of bad ones, they don't really have the "oomph" to get into the thick of things and have an uncanny ability to get one or two-shotted.
That being said, agi builds are more fun to play imo, but a whole team of agi builds just won't cut it.
And this is where the balanced build comes into play. A good player will do best with a balanced build since he can deal with almost any situation by playing to his opponents weakness. You're a str build? I'll drag you out in the open, harass you, outduel you, outrange you and I have the oomph to do this in a manageable amount of time. You're an agi build? I can keep up with you. I'll play defensively, capitalize on stuns, force glances and then quickly 1-2 shot you.
Specialization is detrimental to a good player, you want to be as versatile as possible.