You realize, that no one in the series has more than one type of magic, right? Why should Lucy, and Lucy alone, learn two types of magic, which would make her primary magic suffer? Its the question of being a specialist vs. a jack -of-all-trades, and generally, specialist wins outs.
She's a summoner, her training is to be able to sustain as many summons she can for as long as she can, the summoner's primary battle strength, is that they can bring an army to a duel. I have no doubts, that by the end of the series, she'll have all 12 (13?) spirits out at once fighting at the same time. You also forgot, that she's been practicing the ultimate Stellar Spirit mage spell.
When her keys or summons don't get "lol-plot"-ed, her magic is really powerful.