Simon's BASIC was buggy. Some worked great some didn't work well at all (like line renumber) IIRC there was an updated version out there..

Second, I think the power of the 64 was its' limited BASIC but it was a good start (still the best line editor of most systems, just cursor up change the line, press return and done), The no-frills in the language but not in the capabilities made it a platform where there was no one definate way to do graphics or sound so there was a lot of experimentation most good, and some downright awesome. I think not having those features baked in the ROMs made the community a lot more dynamic.