Heh, Lucy/Summoner-type magic sucks because she/they can't tank bosses... Sounds like every MMORPG I've been on that had summoner-type classes. Duh, of course they suck for that sort of thing - That's what the tanks are for (Erza, Elfman, etc).
The top twenty in Fairy Tail all are capable of taking on entire guilds now - yes, even Lucy. She took out Naked Mummy on her own, in canon no less. Fairy Tail's power scheme is NOT the norm for the average power of a guild - they are waaay above it, along with Lamia, Pegasus, and the others. If you want to say that the Celestial magic sucks, that's cool. Just because it doesn't score as highly in mook kills and boss kills as Erza's 'The Knight' or 'Dragon Slayer', or even 'Ice Alchemy, it doesn't make it a 'strong' magic. I mean, who in their right mind would race an SUV, or even a truck, against a Camero...? So...what's the use of bashing SUVs in a racing environment?
Long and short of it all - Celestial magic wasn't made for taking down Guild Master-lvl or Saint-lvl opponents, so it's pretty useless to compare it to magic that can. You're going outside the scope of its intended usage and for Lucy's intention in using her Celestial Spirits in the first place. I bet a fair number of people knew this already.