The Symantec/RapidSSL certificates that WHMCS MarketConnect offers are way overpriced by today's standard, even with the reseller pricing. Products like PositiveSSL are much cheaper. On the basic DV single domain, it's like $10 vs $4-7 which isn't a big deal. But for higher-end products like wild card SSL (which I actually do need) the price difference is like $50 vs $95. For EV it's even worse.
If WHMCS offers the Comodo/PositiveSSL lineup with decent pricing instead of the Symantec stuff, then I'm sure no one would have a reason to use other vendors' modules.