It's not so much specificall the Ash Wednesday practice that bothers me, but the general ecumenical idea that we can focus on things we agree on and downplay our theological differences like they aren't that important. I think we need to be very careful about that. Perhaps the intentions are not to weaken each other, but I believe that will be the results. The differences are very fundamental.
“These words spake Jesus, and lifted up his eyes to heaven, and said, Father, the hour is come; glorify thy Son, that thy Son also may glorify thee: And now I am no more in the world, but these are in the world, and I come to thee. Holy Father, keep through thine own name those whom thou hast given me, that they may be one, as we are. Neither pray I for these alone, but for them also which shall believe on me through their word; That they all may be one; as thou, Father, art in me, and I in thee, that they also may be one in us: that the world may believe that thou hast sent me. And the glory which thou gavest me I have given them; that they may be one, even as we are one: I in them, and thou in me, that they may be made perfect in one; and that the world may know that thou hast sent me, and hast loved them, as thou hast loved me.” John 17: 1, 11, 20-23 KJV
The Church was one for the first thousand years. Now there are over 30,000 different denominations. Since Jesus specifically prayed five times, in this particular passage, that we all be one, it must have been very important to Him and must grieve His heart terribly that we have used our free will to become so separated and fragmented.