IMHO "abuse" does not fit well, since I would call something "abuse", if the player gained an unfair advantage. But...
- this is unlikely, as others pointed out
- Unless the character becomes an Uber-character, it is not problem for the other players if one character uses the rules to get an advantage.
The main question is: Is it still fun for the others players if they travel with such a character? And in this instance it may be that the multiclassing character pisses of the others, because he cannot do anything right, and other's characters players die because of the stupid mulitclassing. In that case I would say it is no good to build such a character.
If, however, all are having fun with such a special character in the group and they still can manage the challenges, then everything is fine.
I just wonder, if the player has as much fun with his character as he would have with a "normal" one. If this is also yes, cool.