There are utility skills but due to either the costs,activation,recharge or scaling some are hard or simply impractical to use. While many professions have useless skills I suggest that the balance team goes in and changes the values or function of some skills in these lines.
An improvement would surely give assassins an extra dimension, where as currently those assets that would much be loved are out of reach due to being ineffective or unreasonable for the amount of investment needed.
Some of the skills are not bad at all, but wouldn't mind a boost.
Sure every profession has it's unused/crappy skills, but thats no reason not to suggest

Suggestions for skills.
Entangling Asp - change to "Spell. Inflicts Poisoned conditions (5...17...20 seconds). If this followed a lead attack the foe is also knocked down and this skill recharges 50% slower " recharge reduced to 10. Essentially so that if they meet the conditions for the knockdown the recharge is still 20s, but otherwise it's just a poison skill on 10 recharge. The skill itself wouldn't surpass other means of poison. The ranger poison skills such as Apply Poison and Poison arrow are much easier to spread and cost less energy, while necromancers poison last longer and usually comes with other affects as well.
Enduring Toxin - an extra pip of health degeneration and apply to adjacent foes.
Blinding Powder - Can't this be 15 recharge? The skill does need to follow an off-hand attack
Mantis Touch - Couldn't this be a touch skill at 8 recharge? instead of a ranged spell at 15 recharge? It would fall in line quite nicely with the other mantis skills, sporting a good cripple duration and no intricate requirements other than a preceding lead attack.
Shameful Fear - one second cast.
Shroud of Silence - Function change? For 4 seconds, the next time target foe uses a spell, that spell fails and takes an additional 5...15...17 seconds to recharge." recharge reduced to 20s. The skill then becomes basically diversion but for spells only.
Siphon Strength - Scale it so the damage reduction starts as 14..41..50 instead of 5..41..50. The changed scaling makes it easier to fit on the bar if it's more effective for less investment.