I would say let your keyword research do the talking. The tried and true method is to use your "seed" keywords, gather all the suggestions you can, measure which keywords hold the most value for you, and look for the opportunities. If one of your best opportunities has prepositions in it, there is your answer. I'm willing to bet you'll find valuable keywords that use them.
My observation is they don't carry much weight, but with voice searching on mobile, searches can be much more "conversational" when spoken into Siri or Google voice search. We tend to keep things short when we have to type it in, right? There is also Google's Hummingbird update, semantic search and natural language processing. WordStream did a great blog on this right after Google's Hummingbird update. I highly recommend you read the whole article. Here's an especially relevant quote:
"You only have to look at how accurate Google Now has become since its introduction to see that natural language processing is going to remain a major part of Google’s plans for search."
Overall, I'd use the prepositions. Natural language is important, sounds better, and the data is probably there suggesting you should keep it.