How is "their angels always behold the Father in heaven" a warning? Matthew 18:10


  Take heed that ye despise not one of these little ones; for I say unto
  you, That in heaven their angels do always behold the face of my
  Father which is in heaven.


Jesus was talking about not offending the little ones who believe in Him. Then He gives this cryptic statement as an added incentive to beware. 

In what sense is this statement a warning to his disciples? Clicking on the link will give you fuller context of the quote. Any Protestant and Catholic perspective is welcome.
Commentator Albert Barnes has some thoughts. First he points to Hebrews 1:14. The entire verse and his commentary are worth reading but here are some excerpts.

Are they [angels] not all ministering spirits, sent forth to minister for them who shall be heirs of salvation?
Are they not all - There is not one of them that is elevated to the high rank of the Redeemer. Even the most exalted angel is employed in the comparatively humble office of a ministering spirit appointed to aid the heirs of salvation. &quot;Ministering spirits.&quot; A &quot;ministering&quot; spirit is one that is employed to execute the will of God. The proper meaning of the word here - λειτουργικὰ leitourgika - (whence our word &quot;liturgy&quot;) is, &quot;pertaining to public service,&quot; or &quot;the service of the people&quot; (λαός laos) and is applied particularly to those who were engaged in the public service of the temple. They were those who rendered aid to others; who were helpers, or servants. Such is the meaning as used here. They are employed to render &quot;aid&quot; or &quot;assistance&quot; to others - to wit, to Christians. &quot;Sent forth.&quot; Appointed by God for this. They are &quot;sent;&quot; are under his control; are in a subordinate capacity.
...
The following methods of angelic interposition in behalf of man are noted in the Scriptures: (1) They feel a deep interest in man. ... (2) they feel a special interest in all that relates to the redemption of man. ... (3) they appear for the defense and protection of the people of God. ... (4) angels are sent to give us strength to resist temptation. (5) they support us in affliction. ... (6) they attend dying saints, and conduct them to glory.

That's just background. Here's what he says on the verse in question:

Do always behold the face of God - This is taken from the practice of earthly courts. To be admitted to the presence of a king; to be allowed to see his face continually; to have free access to him at all times, was deemed a mark of special favor 1 Kings 10:8; Esther 1:14, and was esteemed a security for his protection. So, says our Saviour, we should not despise the obscurest Christian, for he is ministered to by the highest and noblest of beings by beings who are always enjoying the favor and friendship of God.

The Almighty God is a loving shepherd and merciful father to his little ones, and a general to an army of angels. Cross one of his little ones and you'll endure his wrath.