Look at it this way. Lets say I give you 5 characters to create a password. If you use all numbers then each value can be anywhere between 0 to 9. That's only 1

Look at it this way. Lets say I give you 5 characters to create a password. If you use all numbers then each value can be anywhere between 0 to 9. That's only 10 values! So that's 10 to the power of 5. That's only 100,000 permutations. With letters, assuming standard English, you get 26 per value. Or 52 when we include case. That's a big difference. So lets say I only give you 4 lowercase characters. That's 4 to the power of 26. 456k permutations. So almost 5x more secure with less characters! Your offline password cracker can crack numeric only passwords with ease. Heck, if you know who supplied the access point, you can narrow your search. For example I have uverse and I got a 10 number password by default, just like all uverse customers. So if you know its a uverse customer you can tell your cracker to focus on 10 digit permutations. There's only 10 billion permutations there. With 10 lowercase letters its 141 trillion.