Manual extension tubes can be useful with any lens, as long as it has an aperture ring. I'll give you two examples, both taken today at lunch, and both using an external flash on the hot-shoe. My extension tube isn't a nice set - mine started out in life as a useless 2X teleconverter that my father had bought a number of years ago. It was very poor quality, so (on Roy's advice), I took a hammer to the glass and now have a useful extension tube.
First, here's a picture taken with my Vivitar Series One 105 macro lens set to 1:1. Aperture will be available in the picture.
This picture is full frame, only resized down and a little USM. No cropping at all.
Here's the same subject, shot with the same lens still set to 1:1, only I added the extension tube. I used f16 (I think, since I had trouble with having enough light for f22, even with the flash), and had the flash set to the "A" mode.
This is also the full frame picture, no cropping at all, just resizing. As you can tell by comparing the two pictures, it is more than 1:1. So even though your set of extension tubes are manual, they are quite capable of producing good images (with some practice focusing!).
Some day I'll get a tripod and perhaps macro focusing rails for it. Are the rails worth getting?