Just to mention, that comparison of glass of milk with protein drink is obviously stupid. Sorry but it is. How much protein is in 1 glass is shown on every packing I ever saw (usually 3g/100ml).
Please read about them (http://en.wikipedia.org/wiki/Protein
), proteins are composed of amino-acids, our body knows about 20 of them. Only and only when they are composed in food during digestion in specific amounts body can digest & use them. Otherwise it just flushes them down the drain.
So Broccoli has 50% of protein content. Great, right? Nope, the mixture is very bad. Similar goes for Soya. There are ingredients that contain good mixture, like white parts of eggs, whey etc. That's why drinks are made from these things.
To OP - if you don't want to add muscle mass, I don't get why are you even asking about protein supplements. Just keep doing what you do.
Saying that normal person needs 2 g of protein per kg is very harmful advice, especially for liver and kidneys. In longer term, this intake might damage them. Unless you are aiming for world-class bodybuilding competition and you train accordingly, don't go over 1-1.5 g per kg, depending on how intense your workout is.
Another story - I don't think with usual office guy working out in evening, protein intake should be counted per day. What I mean - when you eat lunch with 30-40g of protein, I am not sure how much it helps in recovery of workout done 20:00-21:00 for example. Most of things have already moved on in digestive system, let's say beyond reach of body
Just my 2 centimes...