Heirloom tomato is a confusing term to some people and there are more than one type of heirloom tomato. There are Family Heirloom Tomatoes that are usually seeds that have been passed down over several generations and where saved for their special qualities. There are also Commercial Heirloom Tomatoes these are from plants that are open pollinated and where grown commercially for sale before 1940.
Family heirloom varieties are disappearing as more and more small farms are lost so to is the genetic variations of those family heirlooms. These seeds were saved from the best producers over many years the weaker traits were breed out by selection of the best. Each time a new season was over the plants that had adapted best produced the best and were most vigorous were saved thus allowing these varieties to survive for 100s of years. Now the call for heirlooms is not so great although in the past few years they are again becoming popular the majority of the tomato seed grown today is for hybrid tomatoes which have longer shelf life, hold up better to handling and shipping, they just do not have the flavor and texture that old time tomatoes have though.
I grow heirloom tomatoes myself and at first I found it necessary to purchase some plants of the two varieties I grow Cherokee Purple and Black Krim, but after the first year I saved seed from both of these plants and the next year I grew three of each plant. At harvest in the second year I saved seed from the best plant one from each variety. Last year I did the same thing and this year I will again plant three of each and save seed from the very best ones. I actually have seen an improvement in these tomatoes from the first year. I know the weather plays a part but I think it is worth it to sacrifice a couple of tomatoes each year to be relatively certain that the following years crop will be good or better.
While commercially available heirloom seeds is perfectly fine and will grow good quality fruit for you the real secret to a good heirloom believe is that it has adapted to the soil, weather, and other growing conditions of the local area. In order for this to happen it may take a few or as many as eight years of seed saving to get localized heirloom tomato seed. I guess it is all a matter of personal choice on how far you want to go with growing heirlooms. If you have not grown heirlooms in the past I would encourage you to try some this year you will be pleasantly surprised and may just get hooked.