Shark Bite fittings have made certain types of plumbing repairs much easier to deal with. Most local hardware stores have various sizes and types in stock. Shark Bite fittings definitely have a place in the plumbing industry. However, this does not mean that they can completely eliminate the need to run copper.
I have seen shark bites on heaters go bad. It may have something to do with expansion and contraction that can occur at the hottest point.
As a licensed plumber, we have added many bathrooms to existing homes that have copper pipes. An easy solution to adding a line would be to use a fitting that would allow you to run a new pex line off the copper. This is a quicker repair but I will only use this as a last resort. By installing a copper fitting and then reducing to pex you are making a permanent connection that can not be pulled apart.
When installing a new shower valve we always mount the valve body around at least 12″ of copper and attach it properly to a support. It is important that the valve body be properly attached in case the unit ever needs to be serviced.
Many plumbers will avoid using compression type fittings as part of any plumbing installation. The reason for this is because they are the ones who have to repair leaks from previous installs. Installing risky connections behind walls can be like a ticking time bomb. Chances are nothing will happen but if it does it will definitely be costly.