I always wondered, why the native EQ often sounds a kind of thinner and/or less present compared to a few other EQs, when using their high and low shelfes. I meanwhile got aware of the fact, the frequency set in the native EQs is, like for the EQ bells, the center frequency of the shelf-curve. For the EQ bells this of course is the way that makes most sense, but it imo doesn’t for the shelfes. Dependend on the Q (in the EQ on the shelfes the real Q-factor is used, not the width in octaves, like on the bells) and the kind of shelf you use, the EQ currently affects way earlier on the bands, than the user probably intends to affect.
1st example:
When I currently use a low shelf at 100Hz with a Q-factor 1.0, the actual shelf curve starts at ~162Hz (respectively the shelf curve starts at ~62Hz and ends at ~162Hz), because the 100Hz are just the center mark of the shelf curve. The result is a thinner (or overboosted) sound on the lower bands.
2nd example:
Currently using a high shelf at 12500Hz with a Q-factor 1.0, the actual shelf curve starts at ~7725Hz (and ends at ~20225Hz), because the 12500Hz are just the center mark of the shelf curve. The result is a way less (or over-) present sound on the upper bands.
Of course it’d be right, if someone would say now “Well, then simply adjust your frequency settings.”. I just doubt many people are willing to throw everything they learned about mixing theory overboard and/or start handish frequency-calculations.
Editorwise setting the center frequency of course absolutely makes sense. If you’d set the top frequency of a low shelf and just change the Q-factor, your frequency setting would already be screwed and you’d have to re-adjust. So making the top or bottom frequencies of the shelfes a parameter would instantly become pretty annoying. Instead it imo would make a lot of sense, be a real enhancement and really really useful, when the EQ would calculate the top and bottom frequencies of the shelf curves and display them in realtime during editing. This way it’d be possible to really optimize your EQ shelfes by only having an eye on the frequency display, while editing your actual EQ settings.
Edit:
And reading this again, it doesn’t actually make a difference, if I adjust a center frequency or a top/bottom frequency after changing the Q. 
So maybe it’d indeed be the better, more simple and more sensefull idea, to make the actual EQ frequency parameter for low shelfes the top frequency of the shelf curve and for high shelfes the bottom frequency of the shelf curve. Internally then just the center frequency would have to be adjusted/re-calculated.
