Zyban® and Wellbutrin SR® (both bupropion HCl) are sold in a sustained-release formula not because bupropion can be hard on the stomach, but because the original, immediate-release formula caused a higher incidence of seizures than other antidepressants. (The original Wellbutrin was introduced in 1986, I believe, and withdrawn a year or two later.) Burroughs-Wellcome (later Glaxo-Wellcome) reformulated the tablet in a sustained-release formula, which reduced the possible occurence of seizure to an acceptable level (i.e., comparable to other antidepressants).
However, Wellbutrin SR had a hard time re-establishing itself as a first-line depression med because the name was still associated with seizure. (A shame, because IMO, it’s a highly-effective antidepressant, especially for lazy depression, as it’s rather stimulating.) As well, by that time, Prozac® had made its splashy debut.
Gradually, Wellbutrin SR taken its place as a widely-prescribed antidepressant (and Zyban as an aid to smoking cessation), but I doubt many GPs think of it as first-line treatment (though psychiatrists would be more open to the idea). A small dose of Wellbutrin SR added to an SSRI such as Paxil® has been shown to reduce or reverse the incidence of SSRI-induced sexual side-effects. (Wellbutrin on its own doesn’t really cause these kind of side-effects; in fact, it can improve low libido.)
More info here: http://www.wellbutrin-side-effects.com/wonderful-wellbutrin.html
So, back to the point of the OP, to crush Wellbutrin SR tablets would turn them into regular Wellbutrin, releasing all of the medication into the system at once, rather than more gradually - and the risk of seizure would be greater.
A side note: Wellbutrin XL® - a once-daily dosing - is now available in the U.S. The extended-release formulation has been improved. GlaxoSmithKline (formerly Glaxo-Wellcome and SmithKlineBeecham) wanted to release it under a completely different name, to escape the “risk of seizure” curse associated with Wellbutrin, but the FDA said no.