Ray,
T I T I T I T I .....and so on. Thumb lead makes it difficult to get melody notes on the 1st string. You can reach higher on the 2nd string to get a thumb note or, as some do, you can get it by reaching the thumb up to the 1st string.
Tom Sauber switches between thumb and index lead. Pete Steele did this too.
Index lead allows you to comfortably use your index (lead finger) on strings 1,2,3 & 4. Your thumb likewise can easily reach strings 5,4,3&2. Having your index, which is going to be picking the melody notes, comfortable on 1,2,3 & 4 opens up all sorts of possibilities. You can learn to get your thumb onto the 1st when needed and your index on the 5th as well.
The sequence with drop thumb is I T I T I T I T. As with any style, you can add left hand devices where you need them.
Hope I haven't confused the issue!
Chip A.