Worn down Front bottom teeth due to Grinding - Dentist want to do filling with composite?

I disagree with that recommendation. It might work in the short term, but in the long term I would expect failure.If your natural teeth have worn down from grinding, then the composite filling will either wear down as well or break off. My opinion is that you would be better off with porcelain veneers, which won't wear down. One of the stronger porcelain veneer materials is EMax porcelain, which I use a lot of the time and would highly recommend to you over composite bonding on your worn down lower front teeth.

I agree entirely with your dentist's recommendation.  Keep in mind that dentin wears 9 times more rapidly than enamel, so it is paramount that you placed a protective layer on the edge of the tooth for prevention.  The beauty of composite bonding is that it is minimally invasive and reversible.  If it is uncomfortable, or it wears or chips, your dentist can adjust or rebond it.  I personally would not place a porcelain veneer or especially a crown because they will require more tooth preparation.  On the other hand, if there is further structural damage of the tooth other than incisal wear (edge wear) such as decay or a deteriorating filling, in that case I might want to consider a veneer or crown. 
That being said, I would thoroughly plan for this prior to treatment in order to get a predictable, long-lasting result.  What I mean by that is have your dentist come up with a diagnosis and a waxup prototype in order to see where the teeth and material should go prior to treatment.  At our office, we don't just treat, we also investigate why certain things happen.  If the teeth are worn, why is this occurring?  It could be neurologic, stress, or there could also potentially be an airway issue or underlying sleep disordered breathing that has not been identified.  Research has shown there is a correlation between sleep/airway issues and grinding.  Since there are no photos or xrays, I am guessing that there may be secondary eruption or dentoalveolar extrusion in which the lower teeth might have erupted, or moved up towards your upper teeth.  Does it look like there is a "step down" from one of your lower front teeth down to the tooth behind it - canine/premolar?  Your dentist would have to make sure there is enough room when you bite and grind for restorative material such as composite so that it will last a long time. If there is lack of space, you can do 1 of 2 things:  orthodontics to intrude/move your lower teeth down into the gum, or gum treatment to expose more tooth and have a larger surface area for bonding.  Again it all starts with planning by your general dentist.  You might want to consult with an Orthodontist, Periodontist and a Board Certified Sleep Physician as well.  I would also consider wearing a custom-made nightguard for added protection.  Again these are all suggestions and it may sound overwhelming, but I strongly believe to go through a checklist and cover all your bases - better safe than sorry.  Hope this all helps, good luck!  

In todays fast paced and busy world, it is very common to see patients just like you who grind their teeth. There are a few options for correcting worn teeth and protecting them. One option is composite bonding. This is the most conservative and most economical. It will protect the dentin. The limitation of composite bonding is in the aesthetics. The tooth cannot be lengthened or reshaped significantly with this technique due to risk of breaking off the composite. The teeth will be the same length which is usually shorter than ideal in patients who grind. A second option could be porcelain veneers. This would allow the teeth to be protected and can significantly enhance aesthetics to improve contour, length, shade, and alignment issues which are common with patients that grind. Veneers are thus aesthetically superior, but are more expensive and require slightly more tooth structure to be removed. A third option for some patients are crowns. Crowns are recommended to severe grinders who are not candidates for composite or veneers due to the amount of force they place on their teeth from grinding. Crowns are equal to the aesthetics and cost of veneers, but cover the entire tooth.Regardless of which option is chosen, a nightguard is essential for success. It will protect your teeth and the restorations. It will allow them to predictably last long term and will help to limit future issues.

I see this problem all the time.  There is no one answer for all people because there are a variety of causes and remedies for people who grind their teeth.  To answer your question bonding these teeth with the proper composite will retard the wear process and often strengthen the teeth.  It is not a permanent solution.  Now that people live such long lives this problem and solution becomes even more complex. You might consult a dentist who is an expert in 'occlusion' and explore the causes for your grinding.  In the meantime the bonding will be okay.

My concern about focusing on these 4 teeth when it is established that you are a grinder is a lack of context.  Are you 24 years old or 74 years old?  Why are you grinding, is it a byproduct of uncontrolled sleep apnea?  How are the condition of the other teeth in your mouth?  It is easy to suggest simple to complex solutions for what you have described, but I have found that the best long term solution is understanding why a problem has developed and addressing the cause in the solution.  Without more information I am reluctant to suggest you have the treatment done.  I suggest you see someone who will take a more broader diagnostic view.