Why does the Messiah have to be God? It seems to be incredibly important to numerous "brands" of Christianity that Jesus be God. So much so, that traditions that reject the deity of Christ are often dismissed as heretical by orthodox Christians.

Why is this so important? Why couldn't the Messiah just be a man? 

The most common reason I here is that the sacrifice had to be perfect. The Messiah had to be God because only God is perfect.

And yet (some? all?) Catholics believe that Mary was also perfect. Why couldn't she be the sacrifice? If God could make a regular human being perfect without them  being God, isn't the Christ's deity superfluous?

Is there a deeper reason that Jesus had to be God that is compatible with the sinlessness of Mary?

EDIT: In an effort to clarify...

I'm just wondering why the Messiah had to be God. Some people say it's because he had to be perfect. But if the Catholics are right (if Mary was sinless) then God could have just created the savior sinless (without him being God.), so I'm wondering if there's a deeper reason that the Messiah had to be God
I think that there are two main reasons why the Messiah had to be God.  

One, which you alluded to in your question, is that he had to be sinless in order to be the perfect sacrifice for sins.  I don't hold that Mary was sinless (Romans 3:10), and I believe that only God could have been sinless.  So in my opinion, that is a valid line of reasoning for the necessity of the Messiah to be God.

The second reason, is that the Messiah needed to be God for the same reason that he needed to be man.  Hebrews tells us why Jesus had to become man:


  Hebrews 2:17-18 (NIV)
  17&nbsp; For this reason he had to be made like them,  fully human in every way, in order that he might become a merciful  and faithful high priest  in service to God,  and that he might make atonement for the sins of the people.  18&nbsp; Because he himself suffered when he was tempted, he is able to help those who are being tempted. 
  
  Hebrews 4:14-15 (NIV)
  14&nbsp; Therefore, since we have a great high priest  who has ascended into heaven,  Jesus the Son of God,  let us hold firmly to the faith we profess.  15&nbsp; For we do not have a high priest  who is unable to empathize with our weaknesses, but we have one who has been tempted in every way, just as we are  —yet he did not sin. 


The Messiah needed to save mankind, and in order to do so, he needs to act as a high priest, an intermediary between God and man.  These verses show us that Jesus needed to become man in order to empathetically mediate for man.  I suggest that he also needed to be God, in order to mediate to God.  Jesus acts as a bridge between God and man, by having both natures.

Because Jesus is in nature God, we have a perfect high priest in the heavenly tabernacle.   This provides for us an anchor behind the veil (Hebrews 6:19), which is far superior to any merely human priest.  Hebrews 7 explicitly states the flaws in having human priests, and why a priest with the eternal nature of God is far superior:


  Hebrews 7:23-25 (NIV)
  23&nbsp; Now there have been many of those priests, since death prevented them from continuing in office; 24&nbsp; but because Jesus lives forever, he has a permanent priesthood.  25&nbsp; Therefore he is able to save  completely those who come to God  through him, because he always lives to intercede for them. 


This is contrasting the disadvantages of human priests with the advantages of having God as our high priest.  The writer of Hebrews then goes on to list some characteristics of such a high priest that meets our needs:


  Hebrews 7:26 (NIV)
  26&nbsp; Such a high priest  truly meets our need—one who is holy, blameless, pure, set apart from sinners,  exalted above the heavens. 


In order to intercede for us to God, the writer of Hebrews tells us that this high priest needed to have many characteristics that we would only associate with God: holy, blameless, pure, set apart from sinners, exalted above the heavens.

Even if we accepted the sinlessness of Mary, I don't think anyone would attribute to her these necessary characteristics of an effective heavenly high priest (except perhaps pure and blameless).