I personally don't like using legends...however came to the depressing conclusion that my team would have a huge opening to dragons without Jirachi. I personally hate using legends even if they're not ubers, but its allowed. Not denying this at all.
No its not justified. Zapdos is an OU Pokemon, not an Uber. Therefore, using Zapdos in standard does not justify him DC'ing on you.
Unless of course it was a BL/UU battle, territories where Zapdos may not be used.
EDIT: NOT ALL LEGENDS ARE UBERS.
Suicune
Raikou
Entei
Articuno
Zapdos
Moltres
Jirachi
Celebi
Phione [lol]
Regirock
Registeel
Regice
Those are not uber. Ubers are the remaining legends not mentioned.