Then/than has always grated on my nerves, as has there/their/they're. Probably the one that gets me the worst is people who not only get me/I wrong, but also "correct" others who had it right in the first place.

But, on the subject of "prolly" ... it is a word. It can be spelled and pronounced and it has a meaning. That makes it a word, plain and simple. If enough people use it and understand it's use it becomes a proper word and enters the language. Eventually with enough usage those who document the language will include it in their tomes (dictionaries). Note that i said "document", not "define". The people who use the language define it. Dictionaries merely document those definitions; they don't create them.

As i get older i get less hung up on the technicalities of language. Language is meant for the people to use for communication. People are not made to be exemplary users of some particular language specification. Yes, grammar and spelling are important. Note also that misspelling and misuse of grammar are also important, when that facilitates the meaning better than proper usage does.

"Prolly" has a very nice, homey, casual feel to it. If the writer wishes to convey this feeling then using "prolly" accomplishes the job much better than "probably" does.

Note that in my own writing i use lowercase "i" for the first person pronoun. It's not laziness, nor a keyboard error, as is evidenced by the first letter of this sentence. It's a purely personal conviction that others are more important than i am, and i remind myself of that by not capitalizing my pronoun. Is it grammatically incorrect? Perhaps. Does it help me express myself better? Yes. Language is about expression, not about rules.

Was this diatribe necessary? Prolly not. ;)