The reason the LibDems joined the Tories in a coalition was purely arithmetical, modeller. Political persuasions did not come into that equation.
A coalition with Labour would have held 315 seats. This would still have been a minority (only 8 seats more than the Conservatives held alone) and would probably have lasted no longer than the Tories alone would have. A coalition with the Tories was a far more attractive proposition, especially one which immediately declared (unconstitutionally) that they would be tied to a five year term.
Fortunately, as you point out, the unison did serve one useful purpose – that of kicking into the long grass hopefully permanently the preposterous AV system.
The LibDems would get into bed with the Monster Raving Looney Party if they thought they would be given the spare keys to No.10. In the sport of boxing some of the participants are sometimes known as “a nuisance”. They have absolutely no chance of success, but cause problems for the top competitors. So it is with the LibDems. They are a nuisance of the first order and the fact that they are calling the tune in so many important areas of government policy is a scandal.