We can take a look at dictionaries to help us a little. Merriam-Webster's 11th Colligiate has eclectic as:
"1. selecting what appears to be the best in various doctrines, methods, or styles."
Syncretism is defined as:
"1. the combination of different forms of belief or practice."
Both of these practices were found in the ancient world. Both were practiced by our ancestors, whoever they were and wherever they lived. It isn't eclecticism or syncretism that are, in and of themselves, a bad thing. Far from it. What a lot of us are actually concerned about is not eclectic or syncretic practices, but sloppiness and disregard for context.
Context is an extremely important part of any reconstructionist process. Language and culture influence religions and spiritual practices deeply and they help to determine what types of practices develop in different regions and for different deities or spirits. Sincerity is fine, but when you're looking at culture and custom, a sincere mistake may still be an insult and can have some serious consequences. Culture and pre-existing practice determine things as important as acceptable offerings, how one approaches deity or spirit, and often the forms used for prayer or propitiation. What is acceptable in one practice may be strictly forbidden in another, and if you are inviting deities or spirits from many cultures to your table, knowing these rules can make the difference between success and failure in your relationships with them.
In spiritual practices that regard deity or spirit as something strictly internal, this is obviously not going to be all that much of an issue. If it's all in your head, from what I can see, it doesn't matter that much what you do so long as you get the results you're looking for. But from the viewpoint of someone who believes in the external/Otherworldly existence of deity and spirit, small things can make all the difference in the world. Effort is important. History and custom are important. This doesn't mean that nothing can change, but it does mean that knowing the road signs is useful and can often keep you out of troublesome spots along your path. If the mountain is an illusion, the map doesn't matter, but if it's real, you'd best not ignore the place where it says there's a five hundred foot cliff. It makes all the the difference between whether you bring a day pack or your mountain climbing gear.
Both eclecticism and syncretism can be legitimate parts of CR and other reconstructionist religious practices. It's best when those things are approached carefully and allowed to grow organically. Time and depth of both study and practice are necessary in understanding how to enlarge a practice or invite a new deity or spirit into your life. Sometimes they come pounding at the door while at other times they approach quietly and subtly. Often this will depend on the personality of the deity or spirit in question. Sometimes it happens that we decide we need new energies in our lives or wish to cultivate new relationships to help us with goals and areas that are challenges. Like people, each spirit or deity is going to have its own areas of expertise, and sometimes going to a deity from a different culture may be the best answer for us.
It's helpful in these cases to remember that not all our deities have to get along, in much the same way that not everyone in our family or among our friends gets along with everyone else. Uncle Fred's favorite pot roast might upset your vegan best friend Clara's sensibilities. It doesn't mean you can't invite them to the same party, but it's a good idea to have things that they can each enjoy when you're feeding them if you want both of them to be happy at your table. Remember that a favorite offering for one deity might be unclean or an offense to another, and keep those things as far separated as necessary -- different altars, or even different rooms in the house might be a good idea.
Eclecticism and syncretism aren't about cooking everything together in the same pot. They're more like creating a satisfying, multicultural meal that includes favorites and harmonious dishes from many lands, with many ingredients. It takes great skill and a strong knowledge of ingredients to have it all come out right and be satisfying for everyone joining you.
Sometimes the experiments fail pretty badly, but that's a part of what it takes to create a working practice that involves deities and spirits from more than one culture. Be prepared to offer apologies if you've unintentionally offended. It goes a long way with humans and with spirits.
When it works, it can be glorious.