I use a few like this for my boyz instead of the usual buggy, because lets be realistic the current model sucks. I do have a lot of the old plastic war buggies and the footprint is the same even if this one is a bit taller as of yet I have had no complaints from people I have played against.
Obviously this is more of a conversion than a proxy, but WYSIWYG only goes so far - I have both versions of Ghazghkull Thraka if I tried to use the old one in a tourney I would expect people to be annoyed but following strict rules is legal, I also have all the versions of the Ork Dreadnought/Deff dread RT model is again a lot smaller than the newer ones. If I could use these legally, and I will continue to use my big gunz even though I will eventually get new models, is it fair to complain when someone uses something that is recognizable as what it represents?
On the other hand I feel that if you are proxying weapons such as the space marines mentioned above you really have no excuse not to at least mark them in some clear way - this is something anyone who plays Blood bowl leagues or necromunda and mordheim campaigns runs into.