Purchasing of mechs would potentially be unbalancing as not all builds are equal. For example, if only the "primary" designs are available for free, some of the best builds in a given weight class & tech level will only be available to those willing to pay (such as the WVR-5M, MAD-3D, WHM-6K, etc). While this is not an insurmountable advantage, it is an advantage. It only gets worse if real money is the only way to get advanced or experimental tech.
On the other hand, in addition to purely "cosmetic" stuff, there are other things that they can charge real money for. For example, they can limit the number of pilots you can have active at one time unless you're willing to pay for more. As you can only play one pilot at a time, the only real advantages this gives you is the ability to try out more builds at one time and give you more chances to play if there is any sort of down time built into the game (such as time for repairs or to return to base after a deployment). Likewise, they could charge a fee for entry into special events (they just need to make sure such events don't confer any bonuses not available outside said events), such as large-scale operations and the like.
I think there's enough options for non-balance threatening micro-transactions that are still interesting enough to be a draw that they don't and shouldn't go the route of charging real money for mechs.