While I can't vouch for the Imperial Guard, I'll rarely slap a few on my Space Marine Rhino's if I feel I need the little extra punch in the first round or two. Usually if there are ranged vehicles I want to take out or damage right away like a Fire Prism or something so I can get my army moving with less trouble. Otherwise I'll skip out. As mentioned earlier, they're a risk. Usually it's better to save what points you'd end up spending on something more reliable if you don't have anything like that to worry about or spare points to use.