It's not that he's weak. His strength isn't just very usefull in current lannister decks. Their main focus is card/gold advantage and pycell, tyrion, cersei and Tywin are all much better at that. In other houses like Targ, Tyrell, Bara and maybe Stark he shines though. 

Jaime basically gives the Lannister player a 'free' Military challenge when attacking that they can then leave standing to defend a challenge with when the turn passes to the opposition.

If you throw him at a military challenge he wins, you get power, he loses, he is still standing but you have drawn some of your opponents defensive strength away and for every bicon or tricon committed to defend the Military challenge your Intrigue and Power challenges are going to be more likely to be successful.

For a more Nedly explanation, He is basically a flashy light on the wall, where he goes to fight, people pay attention and while they are busy noticing what the Kinslayer is up to the other Lannister plots and schemes are progressing.

Can Ser Jaime be declared an attacker while knelt?

Can Ser Jaime be declared an attacker while knelt?

The requirements for being declared for a challenge are as follows (RR, p28)
 

In order to be eligible as an attacker, a character must have a challenge icon that corresponds with the type of challenge being initiated, and the character must be standing.

 

Jaime's ability allows him to not kneel after, but it doesn't allow him to be declared as an attacker if he's already kneeling.

Sorry if this is a really dumb question, but 'gains renown' means he gains the renown keyword only yes? Not that he automatically gets power from renown simply for participating. He still has to win the challenge. In other wording : 'he only has renown in a military challenge'.

yes, he need to win to gain power, he just gain the keyword "Renown"