Agree with Alick3000, broad match modified keywords would be far more attractive, and as per link values, nothing would be driven by this process.
Using broad match allows you to match a series of keywords to any longtail phrase. Providing the +keywords you use are present, then your advert/adverts will potentially appear.
As per +summer +dresses - from Alick300 - this would match both of your longtail ideas and more.
e.g.
Top 10 highest rated summer dresses Popular dresses for the summer weather.
Where can I buy spring/summer ladies dresses blue summer dresses size 10 dresses with a summer print
etc
As you can see it gives you a much wider target whilst still being specific to your needs.
I hope this helps.
Tim