I used to keep it for healing, but currently I have too many characters to level, so most of time I am below 1000. Although I try to time level ups with need of healing.
I find it best to level up as soon as the amount has been built up for the character I am working on, on the off chance it leads to an increase in damage dealt by their powers/matches. Have a big (and beefy) enough squad to not have to worry about healing through iso levelling.
Unfortunately I don’t have enough tanks to get through a long slog of PvP without needing quite a few heals. The Iso is the gas that keeps my engine going. Maybe that’ll change one day.
Strictly for healing. At least for me, high levels are not vital to success. Just need to be high enough to beat your opponent and many people are still low level.