"... if ALL the Senators are allowed to do is CONSENT to a President's nominees..." Treb

The Constitutional stipulation is:
ARTICLE 2. SECTION 2.
2 He [the President] shall have Power, by and with the Advice and Consent of the Senate, to ... appoint ... Judges of the supreme Court, and all other Officers of the United States ...

If a President nominated a person that the Senate discovered was an escaped prisoner, formerly serving a life sentence for multiple murders, the consent would not be appropriate.

But do you think the Founders intended, or the Constitutional wording means that if a Senator doesn't think the otherwise fully qualified candidate is pretty enough, or if her purse doesn't match her belt, that Senator should reject that nominee?

The People elected the President because they wanted him to make these appointments.
And part of the problem with allowing partisanship to play a material role in it is, that's a two-edged sword.
So if the Dems. block the Reps. nominees, and the Reps. block the Dems. nominees, the judiciary will suffer at the hands of the legislature.
That's not consistent with the "co-equal branch" concept of our three branch government.
""Illegitimate"? Who gets to decide what reasons were or weren't illegitimate ..."

Even if there is no explicit, written code of criteria for this, objective standards can well apply, as they do in courts of law, and in other instances of the confluence of government, and exercise of judgment.

"when the bigots of this world have been privileged for as long as they have, to them equality feels like discrimination." shiftless2