I'm pretty much just agreeing with and amplifying on what Tom has already said. I have made the transition from being a pretty extreme source lumper to being a pretty extreme source splitter, so I see this issue from both sides of the fence.
It seems to me that when a Web site provides you with what amounts to a footnote sentence to use as a citation, the simplest thing to do with it is just to plug it into the footnote field in RM using the Free Form source template. Which is to say, the main purpose of a source template in RM is to construct a footnote sentence for you. When a Web site has already done that for you then you don't need to use a RM source template.
It also seems to me that if you choose to use the pre-contructed footnote sentences that are being provided for you, then you are being nudged pretty hard in direction of being a source splitter whether you wish to be or not. If you wish not to be a source splitter and if you wish to use the pre-constructed footnote sentences that are being provide for you, then you only real strategy is to take those pre-contructed footnote sentences and decompose them into parts with the various parts being plugged into the various parts of the RM source templates. And even if you use the RM Free Form template, to avoid being a source splitter then at a minimum you have to decompose the pre-constructed footnote sentence into at least two parts - one part that goes into the Footnote field and one part that goes into the Page field.
My own strategy is not to use the pre-constructed footnote sentences that are provided for me. As a source splitter, I have RM sentence templates of my own construction into which I plug the various data elements of the citation, and all the data elements are in the Master Source rather than being in the Source Details. It's extra work, but I like the results. I doubt that very many users would like my approach very much. But if you use the pre-constructed footnote sentences as is, you are de facto going to be joining me as a source splitter.
Finally, where I see this whole issue going in the long run is that users will not be copying and pasting footnote sentences in and out of Web sites and will not be using source templates at all. Rather, there will be more of a synchronization process between software such as RM and the Web sites. In this environment, the nudge to use footnote sentences directly as they are provided by Web sites will become even stronger. RM and FamilySearch Family Tree already work together in this fashion if you so choose.