In my 20s I use to prefer no grills on any of my speakers/subs since I thought it was cooler, better for performance and I'm one of those strange folks that generally enjoys looking at my speakers well listening to music, so took me awhile to come around to using grills.
Nowadays at 30, I appreciate the benefits of using grills, saves me having to clean my speakers as often and gives me some piece of mind to protect against general damage, so these days I tend to use them more then not and will only remove them if I've got someone over that I'm showing off the sound system too... though once there off it may be a week or two before I put them back on, sometimes even months haha but for general day to day use I try to keep them on as much as possible nowadays.
When it comes to music though I've always enjoyed watching my speakers well i'm in a musical trace and if my L/R ch speakers had grills id use them for movies and tv viewing and then still end up removing them for deep music listening sessions but with Kef LS50s as my L/R ch Ive got no option to cover them up even if I wanted haha and as much as I love my LS50s and the look of the uni-Q driver, which imo is one of the most attractive speakers on the market... theres certainly times Ive wished they had grills to protect against dust and curious fingers... normal tweeters and woofers arent to difficult to dust with a mircofiber cloth... the tangerine wave guide of the kef tweeter is a pain to dust though imo... but damn it they're sexy looking! so I live with the headache that dusting them causes me haha
When it comes to subs, with them being on the floor I like having the added protection of a grill to protect against unaware or drunk people potentially kicking/stepping into the woofer by accident, though I dont know how much protection the thin fabric sub grills actually would provide against such abuse, it at least gives me some piece of mind knowing the grills are there in the worst case scenario and could just happen to save me some serious pain and heartache.
Honestly If your undecided between grills or no grills a few questions to ask yourself would be... how dusty is your room and how often are you willing to clean and dust the subs/speakers in your system? how much foot traffic is around your subs? do you have children in your household or visiting children often? these type of things tend to be much more critical determining factors then the aesthetics argument alone.
