Why would I NOT use the SQL Server option "optimize for ad hoc workloads"?
The SQL Server development team work on the principle of least surprise - so SQL Server generally has new features disabled in the interests of maintaining behaviour as previous versions.
Yes, optimize for adhoc workloads is great at reducing plan cache bloat - but always test it first!

Kalen Delaney tells an interesting anecdote that she asked one of her Microsoft engineer friends whether there would be circumstances where it would not be appropriate to enable this. He comes back several days later to say - imagine an application that has a LOT of different queries, and each query runs exactly twice in total. Then it might be inappropriate. Suffice to say there's not many apps like that!
If the majority of your queries are executed more than once (not exactly twice); it would likely be inappropriate. The general rule would be to turn it if there are many one-time-use adhoc queries on the database; however, there are still not many apps like that.