In Canada, margarine has to state that it is margarine right in the name.
Generally margarine has too much moisture in it to use for a decorator icing. I say generally because though it isn't recommended I know one or two people that use it with no problem.
I know Kiddiekakes uses margarine, I believe she uses tub margarine in her icing. Her icing is always perfect, so obviously it works really well for her.
Baking and an uncooked unheated icing are two different things. Generally for baking, the best margarine to use is the hard margarine sold in blocks, squares or sticks- as you see in the U.S. The tub margarine has too much moisture for baking although there are some recipes that are adjusted for use with this type of margarine.
Margarine was created as a cheaper alternative to butter, during the war. Both margarine and butter have a lower melting and burning temperature. These fats react differently from shortening or cooking oil when placed on a hot element. When either butter or margarine are used for greasing a pan, the fat can be re-absorbed back into the cake causing cakes to stick to the pan and is one reason why this use is not recommended for most things.
When used in baking, the big difference is taste. If you are finding that it tastes good to you and works well, then always stick with what makes you happy.
Rule of thumb for cakes is that generally butter is better in a chocolate cake, shortening makes a lighter fluffier white cake, butter makes a more dense batter.
With decoratpr icing, all-shortening makes an easier to decorate with icing, an icing with both shortening and butter makes a more flavourful icing that doesn't melt as easy in your hands, an all-butter recipe makes the best tasting icing but is harder to handle and decorate with because of its higher moisture content and the fact that it has an 83F melting point whereas shortening has between an 89 and 99F melting point.
About the only thing I use margarine for other than on toast, is in the odd older cookie recipe I have and then, it is the hard margarine I use. I have tried a lot of recipes using either and I find that I prefer the taste of butter over margarine in nearly all cases.
It is a funny thing, when you switch exclusively to using butter, margarine tastes funny. When all you use is margarine, butter tastes funny. When you use salted or unsalted butter, each taste a bit odd when you switch between the two. I guess ours tastes change and we get used to one thing over another.
Hugs Squirrelly Cakes