There are several things against it. Bernard Shaw has one of his characters in Doctors' Dilemma, I think it was, say that all professions are "conspiracies against the laity." Professional associations are specifically designed for the mutual protection of their members, or there would be no point in setting them up in the first place: their motto is No Entry for Unauthorised Persons. This is not the sort of message that is likely to appeal to the rank and file.
I don't know who first said politics was the second oldest profession in the world, but whoever it was they struck a chord. There was this feeling that you couldn't believe in a politician any more than you could believe in what Milton called "the bought smile of harlots".
The original professionals were not, however, prostitutes but medieval monks and nuns, who differed from modern professionals in not doing the thing for the money. They were given that name because of the Latin profiteor from which the word came, meaning "to declare openly", since they had declared their allegiance to God.
After the Middle Ages the word left the cloister and went public, so that anyone might profess a calling of some sort; indeed, they could profess anything that came into their heads. Professing something was much grander than just saying it, implying a certain openness, a coming clean: the very qualities, in fact, that the New Labour hierarchy is being accused of lacking.
The Prime Minister's spokesman obviously hadn't been thinking of this meaning, but it wouldn't have done him any good if he had, because the old meaning of "to profess" lost its lustre long ago. People who profess their friendship may not be believed: they have put themselves in the category of those who start their sentences with an "honestly" or a "to be quite frank with you ... " They protest too much.
What the spokesman was trying to suggest was that his party had the expertise, the special know-how that professionals have. It was his answer to the charge that the country is being run by a bunch of amateurs, no better than those bumbling LibDems. The trouble is that the British actually like amateurs.
Nor is this an exclusively British trait. "America," said President Woodrow Wilson in 1917, the year in which his country entered the war, "is the prize amateur nation. Germany is the prize professional nation".