For load balancing decisions, sure - I can't imagine many cases where advanced math functions would be necessary. But there are nearly infinite use cases for iRules. In my particular case, I am building a stats logger based on the Link Tracking CodeShare iRule (
http://devcentral.f5.com/wiki/iRules.LinkTracking.ashx). One of the query string parameters my app uses contains a encoded version of the requestor's UserID. I wanted to display the unencoded UserID on that stats page (and also include a hyperlink to our internal CMS for that UserID) and the last step of decoding the UserID is to take the square root of the encoded value :)
Regardless of my particular use case, I can't imagine there is significant overhead involved in including the rest of the math functions. TCL compatibility is touted as a feature of the iRule engine - it would be nice if it followed the spec as closely as possible.