Lets not go down this route eh
if they go together, they go together.
minimix's are done to showcase an artists forthcoming music, there's no way you can compare it.
In general it's nothing you should be ashamed it's just much easier as same artists mostly work with same bpm's so it's just easier to mix thats all. As someone already said; If it fits, it fits. There are loads of All "Artist" mixes and most of them are as good as any other mixes but yeah clearly depends on the artist music and the dj who is mixing it.
Its a perfect example of watching next man, when you should be watching yourself.
Whats worst is when you have a 12 with two different artists on it and you think both tunes would mix really well but you can never do it !