It's not a bad move, just unnecessary. Crabs are pretty easy to build for the Ranged softcap and for constant, ranged AoE and ST chains, so melee isn't needed.
Of course, play whatever you find fun.
I went with a mix of melee and range on my crab - frenzy and arm lash are just too good to pass up and I found that all the mobs tended to close up anyway so I prefer those to frag grenade. I also took the fighting pool (boxing, tough, weave) so I could push my smashing/lethal resistance up and cap all positions with some careful IO slotting.
The cost of this is that I didn't go for a pet build - I never remember to summon the things anyway so I decided to go for more of a tank than a mini-MM.
Globals: @Midnight Mystique/@Magik13
To pick up both the ranged and melee powers? It just seems rather appealing and I'm not attempting to build the greatest crab ever made, nor do I want one that is really horrible. Just one that suits my character concept some and is still effective even if they aren't min/maxed to the extreme.
One of the key things I suppose is that I don't plan on taking a travel power at all, nor picking up spiderlings and I'm not greatly interested in Patron powers other than grabbing one of the AoE immobilizes probably the Mu Electric cages.
This is what I've come up with so far, not to concerned with slots atm until I sort out which powers I'm taking.
1Channelgun
1Crab Spider Armor Upgrade
2Longfang
4Combat Training: Defensive
6Hurdle
8Suppression
10Tactical Training: Maneuvers
12Venom Grenade
14Health
16Tactical Training: Assault
18Frag Grenade
20Stamina
22Mental Training
24Fortification
26Kick
28Serum
30Tough
32Omega Maneuver
35Weave
38Call Reinforcements
41Slice
44Arm Lash
47Frenzy
49Electrifying Fences