I've watched some videos and analyzed Eddy Mexico (very known to spotdodge alot). Luigi got a great spotdodge, with little lag in it that start on frame 2, invincibility end on frame 14 and animation end on 22.
Since Luigi can slide and spotdodge aswell, you can use it as a mixup when you go towards players with shield. Don't overuse it cause players will probably catch on and wait out your animation.
Some examples for use:
Against marth, spotdodge is a great tool cause his hitboxes is not out for long, and against fsmash spamming marth's you can glide right through. (Or you can shield slide and wavedash OOS after hit and punish) He want's to punish your shield slideing with grab, well too bad you spotdodged.
When you miss a laggy move (down B, Smash'es) and opponents wait outside your range to punish, most characters want grabs. Great opportunity to spotdodge if you see them comeing.
Many characters go for grabs at high % for kill (Falcon/Fox/Sheik/IC's) If you tend to spot dodge when you know what they aim for, you can sometimes get more out of sticky situations than you would with shield. Yes I know you can DI sheik's dthrow in PAL, still some shiek's find it valuable to go for.
What are good options out of spotdodge? Since it lasts 22 frames, make sure that you know they went for a move that lasts longer. Even tho people say grab come out on frame 7, the lag end on frame 30.
If you are in range for a combostarting move is what I would recommand at low/mid%, or kill move/setup at high% if that situation occurs.
Hope this helps. I'm pretty new too the scene aswell, so please don't kill me if I'm wrong.