It isn't easy to win a beauty contest against a web-based front end that uses css to create corporate livery and skins for visually impaired users etc etc. As well as showing how easily the customer can include their logo and select fonts etc, the salespeople will also advertise the zero client-side installation/maintenance and high flexibility of such an approach.

It used to be possible to rebut this by showing the advantages of a fat client application such as lower bandwidth use and stronger interactive functionality. Arrival of AJAX and similar is making that more difficult.

If your competitors are gaining ground with pretty front ends, you could probably put together at least some of your screens as dHTML pages so you can do a "me too" demo while also demonstrating your optional "rock-solid interactive Windows forms" for reliability and other benefits, at no additional cost.