I'm going to disagree a little bit with the previous poster and say it might be worth it. After all, it's potted, so you can keep it away from your other plants and wait until fall to plant it. That all depends on the disease, though. Some are more innocuous than others.
Another worry would be the roots. If it's a big tree in a pot, the roots are likely to be a twisted mess, and you'll have to extensively root prune when you plant it.
But...if you have the space and the time to mess around with this tree, depending on the disease, it might be worth it. Most bareroot places ship in the spring anyway, so if this tree doesn't work out, you could trash it and not really be any further behind than you otherwise would be.
Just make sure:
-you recognize the disease the pear tree has, and feel confident you can handle it
-you recognize the chance that, despite your precautions, it may spread to some of your other trees
-are aware a significant minority of store trees are mislabeled as to the variety
-feel comfortable (extensively) root pruning to ensure there's no girdling
Then for $6 I say why not?