Sounds like the sales guy's trying to sell you an extra pair of speakers. There's no reason whatsoever why you can't or shouldn't use the Studio 20s as surrounds. If you plan to use your speakers for both movie soundtracks and 5.1 music, then it's actually preferable to go with direct firing surround speakers.

Dipoles will diffuse the sound in the surround channels, which will help with ambient sound effects, but it will also render directionless the surround sounds that benefit from good stereo separation. IMO, dipoles are a relic from the Pro Logic days when you had monophonic sound getting fed into the surround channels, and movie soundtracks were primarily about mimicking what long arrays of surround speakers sound like. With multichannel music and an increasing number of movie soundtracks, you want the separation and directionality in the surrounds. The way to go with that is direct firing speakers.

The key to making it all work is the placement and positioning. The diagram below shows the reference 5.1 surround speaker placement. You should try to position your speakers in a similar arrangement. Dolby also recommends that surround speakers get raised about 2' above ear level and pointed directly towards one another. This preserves the directional cues, while diffusing the sound just enough to create an ambient effect with more monophonic surround effects.