I apply, disable, and modify definition queries quite frequently in my work. For this reason, I have put the layer definition query selector and the build definition query buttons on my quick access toolbar.
The selector is a bit redundant since the Build Definition Query button also brings you to a screen where you can select a different query, but it doubles as an indication that at least the layer I have selected has a query turned on.
The 'Build Definition Query' button itself, though, has no icon and instead spells out those words, taking up quite a bit of space on the only toolbar Pro gives you:
I think my preferred solution would be custom toolbars (so that space is less of a concern) and something like this idea that allows for easy indication that a query is active on every layer (at a glance, without selection), and one-click access to this screen from the TOC indicator:
But if that's not going anywhere soon, then it would be nice to have a smaller QAT button for those who prefer to work more efficiently/without the ribbon. Thanks.