Usually by the time you are considering re-glazing, the pipes are at end of life, and often the tub itself is starting to blister from corrosion.

I'm not knowledgeable enough to know if the corrosion on these bands means that you're about to have leaks or if it's going to take another 10+ years.

Doesn't seem to be leakage, more like recurrent blockage due to corrosion and the old cast iron piping.  

In this instance excessive corrosion was found and all piping was removed and replaced with PVC.

I have three other units that are already showing signs of corrosion on the bottom seam, so that's 33% of my water heaters that don't even seem like they'll make it 5 years! 

I have seen many fail due to age, either due to severe corrosion of the housing, or, ultimately, the windings of the armature became unwound, due to internal wear.

Also as a landlord you're in the customer service & sales business.Watch out for properties with galvanized steel plumbing since depending on the age corrosion can become a real problem and affect your water pressure.

This scaling also leads to splitting of the pipe due to the expansion of the corrosive material.

If the water's that hard, you're going to be having routine trouble with mineral buildup and corrosion in all your plumbing that you're going to have to be on top of.

Look for corrosion on the joints of copper water tubing to determine excessive flux use?