Effective 1/30/11, Sprint changed their policy so that all "smartphones" (whether 3G or 4G) incur a $10/month adder. Prior to that date, 3G "smartphones" (like the Optimus, Transform, Zio, & Blackberries, etc.) did not incur the $10/month data adder-- only 4G smartphones incurred a $10/month adder.
Sprint's justification for the smartphone adder is that, on average, smartphone users (whether 3G or 4G) use more data, and therefore are effectively more expensive to serve.
If your wife doesn't need a "smartphone" you can still get a data-capable, web-browsing, emailing, texting TV-surfing, 3G phone like the LG Rumor Touch, which does not incur the $10/month adder. The Sprint reps can tell you which data phones do NOT incur the smartphone adder. But they're the phones that are all similar in functionality to the LG Rumor Touch-- which, if I remember correctly, is the best of the bunch, but perhaps not a significant improvement over your wife's LG Lotus.
With the new $10/month smartphone adder applying equally to both 3G and 4G smartphones, I think that it's hard to justify getting a 3G smartphone with Sprint nowadays. I think it's better to either get something like the LG Rumor Touch and avoid the $10/month adder, or spend a bit more up front and get one of the 4G smartphones. Those 4G smartphones are much more powerful than the 3G smartphones, and include Flash support and features like video conferencing, which will probably make the 4G phones feel pretty competent, even 2-5 years from now, whereas the 3G smartphones might be feeling pretty outdated.
FYI: 3G smartphones that were activated on an account prior to 1/30/11 were grandfathered in at the lower monthly fee, on that account (presumably at least through the term of the contract, if not longer). So I got new Optimus smartphones for all 4 members of my family on the day before the deadline (thereby "saving" 4 x $10=) $40/month relative to getting four 3G smartphones after 1/30/11 or getting four 4G smartphones. However, if any of us switch to a different smartphone model (even a different 3G smartphone model) we'll incur the $10/month smartphone adder.