That fact alone makes it quite flexible around blue removal spells
While she makes sense in a tribal context, she was designed to be able to function perfectly on her own, without the need for a board full of vampires
Curses generally fit a type of control build anyway so a control commander in the colors of the curses you want is probably the right choice, at least until The Unluckiest gets printed and we live the dream
One thing not to underestimate about casualty is that only half of the effect will be stoppable by a regular counterspell, which gives the casualty spells in general some added reach in metagames heavy on counters
Mox Ruby gives you Red mana, and has been used in a plethora of decks since they were released
Wizards of the Coast did try to compensate for this by increasing the Mana tax as you increase Esper Sentinels power, but any Auras or buff spells after often reserved for the Commander in Voltron style decks