Couple of reasons. First of all, even with the cheaper available set-up for your gaunts it's going to push them up to 8 points a model, which is pretty expensive considering it gives them no improvement in survivability or killing power.
Second of all, the renewed gaunt unit will come on from your table edge, most likely after the third turn, at which point it will probably be too late for them to do anything to justify the points you spent on the upgrade.