I can understand your point of view.
I also have trained with a myriad of high ranking aikidoka who have or do study other arts. If you take an 8th dan in Aikido who's style has a muscular tinge to it...that may just be your perception, or it may be muscular. Many times powerful ki can't be discerned from muscular strength.
As for people coming from Karate or TKD, I agree they do try to muscle techniques. But, then so does any person new to Aikido. Just because they trained in other "hard" arts doesn't mean they are predisposed or immediately fall into the category of having to be retaught everything.
One last thing. If your training for "budo oriented goals" I think it is wise to be versed in the attacks offered by other styles. This will make you a wiser more effective budoka. And probably would improve the depth of your training.
I do agree a newbie should concentrate on one art until there is some level of proficiency before dabbling. I think this is more important to keeping the arts separated (in one's mind) and free from each other, so as to eliminate the Eastern Taco approach (mish-mash of techniques) that are prevalent in cross-training styles.