How to track touches in a consistent way between touchesBegan: and touchesEnded:?

In some maintenance of some multitouch "drawing on screen" code, I met a bug relative to how references to touches instances should be set between touchesBegan:withEvent:, touchesMoved:withEvent: and touchesEnded:withEvent:.

The code uses a NSDictionary instance to store references of touches in touchesBegan:withEvent:, as follow :

for (UITouch *touch in touches]) {
    NSValue *touchValue = [NSValue valueWithPointer:touch];
    NSString *key = [NSString stringWithFormat:@"%@", touchValue];
    [myTouches setValue:squiggle forKey:key];
}

And retrieves them in touchesEnded:withEvent: this way:

for (UITouch *touch in touches) {
    NSValue *touchValue = [NSValue valueWithPointer:touch];
    NSString *key = [NSString stringWithFormat:@"%@", touchValue];
    NSObject *valueFromDictionary = [myTouches valueForKey:key];
    [myTouches removeObjectForKey:key];
}

In this last bit of code, valueFromDictionary happens to occasionally be nil, as the key is not known by the dictionary. By occasionnally, I mean about 1% of the time.

Now the context is set, my questions are:

1. Any idea why this code is buggy? I'm not sure why it doesn't work, especially because of the very low frequency of errors

2. Some doc from Apple states it's a good idea to use addresses of the UITouch objects as keys : but it means using NSValue* objects rather than NSString* objects ( and consequently using CFDictionaryRef rather than NSDictionary because of the fact UITouch doesn't implement the copying protocol). How come storing a NSString representation of the address rather than the NSValue itself can not be an easy solution to be able to use NSDictionary?<