Using the blockchain for providing other services, besides bitcoin transactions

What I have in mind are proof of existence software, than are a kind a non-centralized and virtual notary service. They time-stamp digital information submitting a hash to the blockchain.

I know already that technically, the blockchain can be used for that. After all, it's basically a time-stamping system.

However, is this kind of service disruptive to the bitcoin community? Can it become disruptive when it reaches a certain level and be curtailed?

I also wonder whether the messages are prunable. Important are the transactions, not the associated messages. If the blockchain grow and grow, would the messages be still be kept?

There are already tons of services doing this, so probably worth focusing your creative efforts elsewhere. Here are the ones we know about on Coin Secrets:

• Proof of Existence
• BlockSign
• Stampd
• La Preuve
• Bitproof
• Stampery
• ProveBit

As for pruning, assuming you use the same OP_RETURN mechanism as these tools, then they may be pruned by some nodes. But I imagine there will always be plenty of full archives of the blockchain available (even if this is eventually a paid-for service) so I doubt that your timestamped hashes will ever be truly lost.