In my projects I use VFOs, DDS generators, and crystal oscillators.

The simplicity of a crystal is a real advantage sometimes.  In other cases, frequency agility and/or synchronization to an outside standard (example GPS) justifies the added complexity of the variable/programmable alternatives.

Even before I purchased the AF4K Crystals business, I maintained a collection of useful crystals for the MF and HF bands.  Whenever I had the notion to repair or or restore a vintage HF rig I was ready with my crystals.  Both commercial and military rigs often use crystals.   Or when the notion strikes me to homebrew a rig from scratch, I was ready with my "box of rocks".  On my website "gallery" page you will see quite a few with crystal sockets showing... and there are many more here that never got photographed!

Don't want to build a collection of crystals?  My website shows a couple DDS VFO projects I built to drive various rigs... 

Remember you need to either key off the DDS or shift the frequency when you are in receive mode or it will interfere with your receiving.   See how the complexity rises?

The QRP Labs "ProgRock" can be a good substitute for a crystal.  It serves as a miniature, programmable oscillator meant to substitute for a crystal.  It strikes a nice compromise between the simplicity of a crystal and the programmability of a DDS generator.  And the latest ProgRock2 version is super small.

The two main reasons people buy my rebuilt crystals is for repair and restoration of vintage ham and military rigs, and homebrew projects.