They are not rare.  I don't know where you are from but get out west to were I'm at (montana) and they were common.  The 5020 was even more popular and the better tractor.  They were big at the time but compared to todays tractors or even tractors that came out later after the 5010/5020 they were too heavy for the hp they put out.  They were basically the last of the dedicated standard wheatland tractors and that is why they were built stout and heavy instead of just taking a rowcrop tractor and modifying it into a wheatland.  We bought a 5010 brand new back in 1964.  Its been long gone but 10 years ago we acquired another one.  The tire (24.5 x 32) was originally designed for the 5010.  And then of course other tractors started using it.  What would be rare is if you found a 5010 that was still original.  The 5010s had a few engine issues and when the 5020s came out JD had an update for the 5010 to turn it into a 5020.  I've heard different stories from different people but I know one guy who got a brand new 5020 engine installed in his 5010 for nothing back in the day.  One of the problems with the 5010 engine was it only held 12 qts of oil.  5020s held 20 qts.  So it would be hard to find a 5010 with the original oil pan.  Our current 5010 has the bigger oil pan.  Lastly, if you think that 5010 is big, look up the 6030.  Almost twice the hp in the same size package and one of the most impressive tractors ever built.