Tracking is difficult now because there are no tools.  If Bitcoin becomes successful as a widespread means of payment, it is a certainty that sophisticated and cheap blockchain analysis tools will be developed both for commercial purposes and criminal purposes.  Similar to the rise of products and tools that currently do comprehensive HTTP log, cookie, and webbugs analysis to track individual customers. 

Imagine in the future a naive user (your mom, your grandma) who is unaware of the technical details, and keeps receiving and sending Bitcoins always using the same and/or chained addresses.  Every time they buy something, no matter how trivial, they are potentially giving a determined snooper/attacker a window into all of their income and purchases since the dawn of time.  This is a disaster waiting to happen.  It will make life wonderful for criminals in countries (e.g. Latin America) where robbery and kidnapping and extortion are common.

It is absolutely essential for the long-term viability of Bitcoin that all clients (and ideally, the protocol itself) have mixing built-in and turned on by default.