the best for defence is all of them combined!!
this is a bad defense for the same reason as it would be a bad offensive force
I suspect that the reason a lot of players have lots of infantry in their hive, regardless of what chassis they are mainly using, is to save up oil.
you can buy 3 infantry armor (the cheapest forces to buy) for the price of 1 tank armor, but if you put those 3 infantry in separate squads on maximum delayed launch you can save up 3 times the amount of oil as you would with 1 tank.
hope this makes sense
just my guess anyway