Why is it acceptable for front end code to be of lower quality than the rest? Your software is only as good as the lowest quality part.
The back end is not. If it falls into the hands of the enemy then it is game over.
Security-wise, it is clearly acceptable for the front end to be of lower quality than the back end.
While I don't think that f/end should be of a lower quality than the rest of the stack, I also think that:
1. f/end gets the most churn (i.e. rewritten), so it's kinda pointless if you're spending an extra $FOO months for a quality output when it is going to be significantly rewritten in ($FOO * 2) months.
2. It really is more fault tolerant - an error in the backend stack could lead to widespread data corruption. An error on the f/end results in, typically, misaligned elements that are hard to see/find.