I could go on and on. Do not always pick the biggest ad in the Yellow Pages. One of the companies in my area that have the biggest ad are the biggest crooks. If your heat exchanger has a hole in it I would say yes. If you had multiple cracks that are very large I would say yes.
Cracks are very common in older furnaces.
They expand and contract with the heat level in the furnace. If these openings are large enough they can force the flame out the wrong end of the furnace and this is dangerous. It is probably not a bad idea. First ask the technician how he determined you had the cracks. Next, ask him to show you the cracks or his determining reasons.