Hi Nicole,
I couldn't agree more with your comments. Particularly if you are doing work for corporate clients where it is so important for them to maintain their corporate identity in everything they do.
They will have specific requirements often for logo size and location, menu style, fonts and corporate colours and for this, slide masters are not only a great idea, they are essential and will save you hours. One tip I would give here is to work out your slide master and then run it past your client at the start for their approval. Not only will this avoid last minute design issues, it is also a good way of making your client feel involved in the whole process right from the start.
The other thing I really like about SL slide masters is the fact that it is not just a single master slide. Instead, you have the ability to make a whole subset of your main master to cope with any subtle changes you might require for a quiz question or a video insert, for example.