I'm fine with it the way it is, honestly, but ... if we're talking theoretically, then the question would be: Is the Dps increase is worth the mitigation decrease?
Firstly, Gladiator already takes cross class skills form Lancer. In fact, before Dark Knight was announced as a Tank, a lot of use were theorizing that Drk would be a Dps job which branched from Gladiator using Lancer cross class skills instead of Conjurer. That obviously did not happen, and Drk was made into another tank.
As things are, Plds would see a pretty hefty Dps increase from lancer skills. Fracture is already a Dps loss, so it's not used. Mercy Stroke is a Dps gain, but the gain is pitiful. Blood for Blood more than makes up for both. Combined with FoF and Sword Oath, that's a pretty big increase in dmg output. Invigorate would also make Pld dps increase over time, because their Tp would actually be sustainable for a change.
The two BIG losses would be Bloodbath and Foresight. Foresight is a weak CD, but it works wonderfully with the Pld kit. It would be a big blow to lose it, especially considering how long the rest of Pld's native CD's are. Un-traited Bloodbath is also not all that great, but it also works really well with Fight or Flight + Sword Oath. Worse yet, if a Pld could use B4B + FoF + Sword Oath all together it would REALLY suck to not be able to take advantage of that extra dmg for self-healing by using Bloodbath.
Personally, I don't think the Dps increase is worth the mitigation we'd be sacrificing ... of course, that's IF SE took away the MRD skills. If we could use both MRD and LNC then it's a whole other story. If we had both, the Pld cross class kit would be: Stone skin, Foresight, Mercy Stroke, Blood For Blood, and Invigorate. I can only imagine what the backlash would look like for SE though. It would be extremely entertaining and ironic to see the wave of Forum posts complaining about how overpowered Pld would be, especially with it's long history of being so broken.