(I'm assuming you mean fighting on foot and not on horseback)

In what light are you asking the question exactly? I mean, it's a valid choice in the way that your knight can determine which weapons he wants to use, but it would've been somewhat unlikely I suppose, especially by the time full plate suits came in swing (very late middle ages).

A spear is not the worst choice for an armoured fighter, but not ideal either. Its advantages are range and relative ease to use in groups, which make them excellent weapons for more or less normal infantry. You don't need years of training to huddle up and point the sharp stick at the bad guys. It's an easier weapon to learn than a sword/axe/hammer/bow.

Not to mention that by this time, many forms of polearms had been developed, like halberds, partisans, bills (originally an English farming tool, it's kind of like a halberd). So if you were a knight looking to use a weapon with good range, chances are that by this time period you would go for a polearm, not a 'simple' spear, as the polearm would not only allow you to stab, but also hack, slash, cut and bash.

As far as 'effectiveness' against plate is concerned, it kind of depends how we look at it. Could a spear penetrate a piece of plate and the underlying gambeson to seriously injure the wearer and put him out of the fight? I would say that is a categorical no. One does not simply 'punch through' a piece of steel armour.

However, as amazing as full plate suits were, they were of course not without their vulnerabilities. There are plenty of weak spots where the individual plates met, creating gaps where you can stab through. Armpits, back of the legs, into the helmet.

Technically speaking, a long stabby weapon like a spear would've been decent for that. But they also had polearms, longswords that were excellent at doing the stabby, maces and warhammers that cared little for armour as they just banged the hell out of the occupant...

so whilst the spear might not be a bad weapon in this context, there existed many other weapons who were either just as good but more flexible in their use or plainly better at dealing with armour. And this is somewhat reflected in the fact that not many historical sources show late medieval fully armoured knights fighting with spears. Longswords, poleaxes, halberds, maces, hammers, yes, quite a lot. Spears not so much. Which leads me to believe it wasn't that effective, otherwise they would've used it more. People back then weren't idiots after all. Those knights would've used the weapons they deemed most efficient.