Arbiters

Well, in the past I never used arbiters. The reason was not because the cost was high (since hell, as protoss, nothing is standing in my way from expanding). But usually is the long stages of development. It seems that its too much of a hassle to get that far. Your goons, zealots with some reavers in shuttles (and hight templars) are enough to hold ground. But one time I lost on a 2v2, or was about to that is until my partner came with a small fleet of arbiters and wiped out both armies attacking my base by recalling and statising units and then moved the attack to the enemies. I guess it all depends. If your skilled you might not need arbiters since your basic units are enough to do the job, they're only there if you really need the extra support.