If you're a noob or a Non-Profrossional ROM-Hacker I have a list of reasons. Becuase I actaully used it with the .SMC file with the most recent version of M3 Workshop.
1] There are 9 headers instead of 13 in the editor.
2] The level offsets were 4 bytes behind the ACTUAL offset after hex FF. NOT BEFORE.
3] The level offsets were incorrect. However Hukka proberly used the MI3 Source instead of by hand. That's why I corrected those offsets in my document in my site or ROM-Hacking dot net.
4] STILL with the NES backgrounds. It dosent matter because I didn't even find the graphic and TSA data yet?
5] Whenever I edit the level, I'm not sure it really sucessfully edit the level or not.
6] Missing level headers won't allow you to change the backgrounds or other stuff. Those kind of stuff can be used with the Hex Editor only.
7] The ***Unknown**** level offsets somehow still the NES level offsets.
Those are my reasons. If you have any more reasons please add.
And believe me. I hacked SMB3 for SMAS way more than anyone else.