Why does Hashem present Avraham's praises here?
Rashi #1: Hashem is saying here - 'Shall I hide My plans from Avraham, who is so precious to Me that he will become a great nation and that all the nations will be blessed through him?'
Rashi #2 (citing a Medrash): Zeicher Tzadik li'Verachah' - When one mentions a Tzadik by name one blesses him.
Ramban: Seeing as Avraham would be a source of blessing for the whole world, Hashem had to inform him of the impeding destruction of Sedom, so that people would not wonder how Hashem could hide such a major decision from him, or why Avraham could be so cruel 1 as to make no effort to save his neighbors by praying on their behalf. Furthermore, revealing Hashem's secret decision to him will be worthwhile seeing as, since he is fully aware that Hashem does justice combined with charity, he will teach his children (and adherents) to go in Hashem's ways. Consequently, he will Daven on their behalf, that, if they will follow that path, they should be spared; but if they don't, in which case they are completely guilty, he will agree that with Hashem's judgment.
Ohr Yechezkel Midos p.104: Is it cruelty not to pray for Resha'im? Yes! This is an obligation of man, and his primary Sheleimus.
רש"י: הואיל והזכירו ברכו: פעמים אחרות הוזכר אברהם ולא בירכו?
גור אריה: כשמזכירו בפניו ומדבר עמו, אין זה נקרא זכירה, ורק כאן הוזכר שלא בפניו.
גור אריה (תחילת פרשת נח): זכירה היא אחרי שיכחה ומכיון שעין ה' תמיד אל יראיו לא צריך להזכיר שבחו של צדיק, ולכן רק כאשר מזכיר צדיק אצל רשע יש להזכיר שבחו, כי הרשע גורם להסתר פנים ושיכחה.