3e Extended Critical in 4e - what would break?

[Calliban]

In 3e's Martial Arts and Compendium II, there was a rule for extended critical hits for high skill levels. 3-7 at 20+, 3-8 at 25+, 3-9 at 30+, and 3-10 at 35+.

This used to make it a good counterpoint to PCs from just spamming rapid strikes back in 3e days, as players tends to do when ganging up on opponent - and we are talking 7+ attacks at skill level 35!

I was thinking on initially only allowing it for a TbaM/WM character fighting a non TbaM/WM character at first. What would be the implications of using this Extended Critical table in our game? Are there any issues I'm failing to see?

My campaign is a Chanbara/Wuxia version of Dungeon Fantasy (but no class restrictions) with high powered PCs.

A side note: We also had a completely insane house-rule requiring a critical defense roll in order to completely avoid a critical attack; a non-critical defense would only reduce the critical attack to a normal hit. This may be why we loved this Extended Table so much. Would our Insane House Rule break something important, other than most of my NPCs?