Basically, you need to ask yourself: are people staying in range of the shield phantasms? Remember that the shield phantasm’s range is very short and only works when people are practically in melee range, and the only reason you’ll be using them is to give alacrity to your team… so if half of them are bearbows or staff Elementalists standing a mile away from the boss, or if you’re against a boss where you move around a lot (Thaumanova Reactor Subject Six / Anomaly, for example), then it’s not worth taking shield.
If you’re sure everyone’s stacking up in range for your alacrity, then the next question you can ask yourself is; do the classes even benefit from alacrity? If your team is full of Revenants, Thieves, Power Necromancers, other Mesmers, hammer Guardians, etc, then alacrity won’t be much of a DPS increase, and you’re likely better off with more damaging phantasms.
Having said all that, in most scenarios, shield is definitely useful to have. Most classes do benefit from alacrity and it absolutely makes up for your loss in personal DPS. Plus, having that extra alacrity helps you too, since your wells and CS will be off-cooldown quicker, meaning more quickness for your party, and quickness is something that every class benefits from.
As for the optimal phantasm usage… 2 avengers + 1 of any other phantasm should be enough for permanent alacrity on your group, but that assumes people stand in your wells. Without shield, assuming you’re on a power build, your best damaging phantasm is sword off-hand, so have three of those out if you’re not using shield. The focus phantasm does OK damage, but it’s used more for reflects (especially if you can get it so that when one phantasm finishes his animation, the other starts, since that allows you to perma-reflect, which is super useful for certain encounters).
Hope that helped.
