Unless you are on a SEVERE sodium restricted diet, the amount you get in a frozen dinner, especially the light ones, is WAY less than you'd think. Do you eat cottage cheese? salsa? Diet soda? Flavored waters? Have you looked at the sodium content on these things?
Example: a half a cup of cottage cheese or 5 Tablespoons of salsa has close to the amount of sodium my lean cuisine lunch did today. (According to fitday)
I've been watching my sodium as well as calcium and fiber recently and I try to stay under the daily recommended amount of sodium by the FDA and I've had to severely change my eating patterns. The days when I'm high on my sodium are actually the days I DONT have a lean cuisine for lunch.
Like I mentioned with cottage cheese and salsa...Breads, cereals, chicken products, any kinds of sauce - soy, ketchup, etc.. Deli meats - anything cured. Here's an example.. 2oz of low-fat turkey-ham (the one with the lowest sodium i could find) came in at about the same amount of sodium as the Lean Cuisine frozen dinner I ate for lunch. (around 500mg sodium) Now, add that ham to some bread.. ugh. Depending on if you have crackers or something else with it or even some type of dressing... you can easily eat half your recommended sodium intake in one sitting.
Anything fast food.. chicken sandwiches (as Suzanne pointed out).. pizza.. even a subway sandwich. I LOVE Subway, but one of their ham or turkey sandwiches is around 1000mg sodium. Obviously the best you can do is to cook your own meals, but I find especially when cooking for other people who are NOT on a diet and might occasionally want something that I cant have... having those lean cuisine dinners standing by is a savior. It's quick and prevents me from picking up something even worse. It's also a great thing for people with portion problems.
They've come REALLY far in the amount of vegetables you get and the quality of said veggies. I used to hate frozen dinners but more and more I'm liking them now. I use them for lunches at work and "emergency" meals at home so I dont have to order pizza with everyone else

(I will say that normally if its going to be my dinner, I pair it with a salad or additional vegetable)