Calculating pi is not really a useful purpose. It has been calculated to 10 trillion (10^13) digits already. Practical use of increased accuracy in even the most precise applications is completely gone after 30-40 digits.
edit: Additionally, the whole idea behind crypto-currency is to have a task that is hard to perform, but easy to check. Finding a block that has certain properties when hashed is very difficult, but once found, it's simple to verify that the result is correct (just a single hash-operation on the found block, which is very fast). There is no way to verify whether the 28389283751th decimal of pi is really 4 or whether i'm just cheating (unless you have a table with all digits, in which case a miner can just use that).
There's a reason why hash-functions are used for all crypto-currencies and simply throwing out the "useless hash-computations" and replacing it with something else isn't going to work.
The basic principle of Proof of Work scheme is - hard to do the work and easy to check that it is done right. There is no easy way to check that newly discovered digits of PI are not just taken from the air, so it will not work.
This
Stop dreaming
