We have been actually noticing that sample texts are turning into the easy temptation to visually enhance an overall composition by adding a fewer elements more (and unfortunately upon identical images already approved), and that explains why so many illustrations fall into the infamous "too simple" rejection reason. They may look nicer, but simply put, sample texts are more of an obstacle than really an added value, both for the submitter, the customer, and why not, for the editor too: Personally, I just ignore (and decline) sample texts over too simplistic works.
See, there are simple illustrations that can be be simple (not simplistic) and still have that sales potential ingredient to become big sellers if the originality and creativity are there, sometimes making our eyes just pop out. Most of them just don't need any sample texts. Ok, of course there are exceptions whereas phrases/slogans (not sample texts) do play an important role in a composition, either over solid or complex backgrounds, but they have to be rasterized images. Final product, with a specific concept, a clear market destination.
The "Editable" statement
Finally, one other deadly mistake on submitting files with vector additional format: The "editability" statement: "fully editable", "resizable", etc.). Layers are not everything, and besides that is a clear misleading message to the customer, so another big No-No. Can you imagine a customer trying to edit a sample text on a vector file? Well, he just won't be able to do that for the simple reason that texts on vectors are to be combined, or converted to curves if you prefer. If not, then they are not full vectorial files, but rather vector files with embedded bitmap images, a known headache issue for customers willing to resize and print them.
Too simplistic files
And finally, those others illustrations: oh well... those who are just too simplistic (not simple, this time), meaning no concept at all, no message, not a simple glimpse of subject matter, etc. Then, there are those popular/oversaturated isolated web icons (Note: 3rd party clipart such as fonts icons, being a big NO-NO as RF)
, originally developed for small size use - and very messy when upsized, web logos, etc. Having said this, please bear in mind that sample texts are not really that important, and not rarely diluting the sales potential of a particular illustration being it a raster/vector/render file. Technical execution, concept, composition and other basic requirements are a whole different story and do play a rather more important role on your work flow.
Hope this helps,