Worth investing in MS SQL?
Not worth it for the reasons you gave, especially considering that you're not going to use it for production purposes. Reporting Services is a great platform for reporting, but I'm sure there are Open Source tools and frameworks to help you with that for PHP / MySQL. Ditto Integration Services, especially in 2012 (much better IDE for developing packages) it's a great tool for ETL, data staging and other import / export work, but probably overkill for you.

I'd definitely stick with the open source for now - it's always going to be cheaper for licencing ;-). Especially as the cost model for SQL Server 2012 shifted to a different per core model. We had to disable cores on recent deployments to bring down licencing costs, as we'd bought hardware before-hand (on assumptions of the same cost model as before). Funnily, enough that's how I started out on SQL Server - with a "we must port this to SQL Server, because Oracle licencing just got too expensive" project.

I look after about 16TB of high-transaction throughput SQL Server production databases for international clients. So definitely not anti-SQL Server and make plenty of use of all those tools you mention.