As Boomhound has already said, whenever you could try to shinegrab outside of shield pressure you could just as easily grab instead. The only possible exception I could think of is MAYBE if you're trying to running shine and you miss and your opponent tries to punish you by grabbing you while you're stuck in shine, you might be able to time the shinegrab to grab them before they can catch you if their grab range is shorter. The amount of time this would take is so minimal though that you would have to expect your shine to miss in the first place to get the grab most likely, and that's not really a productive way to play.
What it boils down to, I think, is that Fox likes to use aerials in neutral quite a bit, and that our opponents sometimes try to beat that by shielding and acting OOS to punish our aerial landing lag. One such punish would be a shield grab. Fox players recognized that when they hit their opponent's shields and landed in front of them, they would be grabbed. To combat this, we started shining in this situation to hit opponents who try to shieldgrab us. Our opponents then catch on to us doing this and choose to hold their shield past the shine that is now expected, and try to punish us then. Following this flowchart of the "meta" of this situation in which fox hits a shield and lands in front of his opponent leaves Fox with, in my opinion, 2 top options: to multishine or to shinegrab. The merits of multishining are that it keeps your opponent in his/her shield and if they're near the edge and you manage to connect a shine on a dropped shield, they will be put in a horrible position below and off of the stage. Fox can generally quickly grab ledge and invincibly shinespike his opponents in this position (especially other space animals). Shinegrabbing is a technique developed to counter our opponents when we anticipate that they will hold their shield through the shine that follows an aerial. Shinegrabbing can be used as an alternative to multishining near center stage where a shine wouldn't necessarily knock a player offstage, or in certain matchups where landing a grab is much more advantageous than landing a shine. It would be good if you knew that a Jigglypuff was going to try to shield your nair approach and bair you OOS, because shining a Jigglypuff isn't always the best thing, but getting a grab on Jigglypuff is about the best you can ask for out of neutral because it leads to upthrow>uair or uthrow>bair as a mixup to catch them trying to SDI out of the uair at high percents or on a stage like FoD where the ceiling is so high compared to how wide the stage's side-to-side blast zones are.
I should mention that if you read that your opponent will hold their shield past your shine and they try to shieldgrab you, your shine will knock them away and if you try to shinegrab here (expecting them to have held their shield) you'll just grab air and they'll be able to reset to neutral during your grab lag. I think this is why we see players like Hax going for multishine much more than shinegrab following an aerial, because shinegrab only covers the "holding shield" option, and multishine covers the "holding shield" and the "acting OOS" options. Fox might be able to shine OOS against multishine and I wouldn't be surprised to hear that there are characters that can shield drop nair between multishines, but I don't know my frames well enough to say for sure.