Yes.
It will likely broaden your technical knowledge, but at the cost of depth. It will improve your communication and people skills and give you a greater understanding of the business and more exposure to people outside of your dept. Your days will be more varied in focus and you will be travelling more. If your total experience is 3 years in design then getting more experience in other areas can't but help broaden your career horizons. But, assuming you have the people and communication skills, it is easier to move from Design to Applications than it is from Applications to Design, particularly from Field Applications work which usually means more face time with customers than a home based Applications job which has more emphasis on the technical side. It is relatively easy to move to Marketing or Sales Engineer from Field Engineer, but extremely difficult to do that from Design. An Applications Engineer is the interface between his company's Design Dept and the Customers Design - and probably Production and Quality as well. You need to have knowledge of the product you sell and the products that it will go into. You may well be helping to define the features/specifications of your companies new products and trying to convince and help customers to make design modification to fit the products you already have.
These titles have somewhat different meanings depending on the company and what it sells, so what I said above is generic. But I speak from experience - having worked in Design and Development, Applications and Marketing.
You already know what design is like, so it all depends on what you want to do now and what are your long term goals.