TLDR: A core aspect of the AST toolkit are their card buffs. An AST can't determine who to give their DPS-oriented cards to if they cannot see the raw DPS values their party members are outputting. Please allow parsers to provide DPS information to assist in one of AST's core functions.
====
One of AST's core functions is to augment their party via buffs drawn from their cards and using these buffs on the appropriate player. While some cards are dependent on the situation of the fight at the time the card is drawn (Bole / Ewer / Spire / Spear), its less obvious which player to give the DPS augmenting cards (Balance / Arrow / Spear in some circumstances). Without a parser and the DPS information it provides, it's difficult for an AST to gauge exactly which DPS are performing well and thus potentially make incorrect choices to whom they bestow their card buffs too.
While it's true you can have a small rule of thumb for card usage, it doesn't mean it's the correct decision in many scenarios. For example:
Give the DPS card to the DPS based on a hierarchy
For single target melee DPS, you can pretty much go most melee will generally be better than their ranged counter parts. However, just because a Job may have higher maximum potential doesn't mean said player is play it at maximum potential. Unbeknownst to the AST, that BRD who's pewpewing hard and fast behind them is outputting the most DPS and they won't know if they can't see their DPS values.
Using Enmity to Gauge DPS performance
With the prevalence of enmity reduction abilities like Quelling Strikes, Elusive Jump, and Shadewalker; enmity doesn't always properly gauge how hard a DPS is hitting a target based on how much that target wants to tear said DPS to shreds. This is especially true if the WAR OT you have in your group is just leading the DPS meter (unfortunately because that implies your other DPS are terrible). An AST would think "of course they would be ahead in enmity, WAR enmity combo is their best combo". So, while reading the enmity gauge can work, it's not perfect by any stretch of the imagination either.
Base it on the ilvl of the DPS
While generally speaking players of high ilvl should have a higher DPS potential, that thought process is totally marred by the varying levels of skill that this game caters too. You can have a DRG in i240 that's being bested by a DRG in i220 because the i240 DRG just isn't maintaining Blood of the Dragon and/or Heavy Thrust in any fashion. Or of course the "I play how I want players" like the fabled Ice Mage or the "I WANT TO STAY IN RAPTOR FORM" MNK. Unfortunately, ilvl isn't a good gauge of card selection because of the varying levels of skill in this game.
-----
Thus I reiterate my original though - please provide an in-game parser just to help AST perform better. If they know which DPS in the party are performing well, they can give those DPS the appropriate cards when drawn and help augment the raid DPS much better. A Balance on a 1,400 DPS is much better than on a 900 DPS and we can only gleam that information if S-E can provide it for us by implementing an in-game parser.